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why do i get the error "Gradle sync failed: Cause: startup failed: invalid token"?

Here's the link to build.gradle . I'm not able to find the error.

I fix your code and build it a success. You can see the PR in Github.
I think it just a little environment setting problem.
https://github.com/kabburesai/Demo/pulls
Update:
I build it again and find the styles.xml has incorrect, so I fix it also.
enter image description here

Unexpected token mostly means syntax error only. There's just a command(code, character or words) that was incorrectly placed.
According to your error, it points to your build.gradle (root).

e: java.lang.IllegalStateException: Backend Internal error: Exception during code generation

At the execution of my android kotlin project on my emulator, I receive this error during the compilation of the project

The cause of the error was i taking my TabLayout's id from xml and just pasted the id in my code without apply any kind of property or calling any function on it.

android data binding: how to get useful error messages

In android studio 2.1, the compile time error messages from xml mistakes are entirely non-helpful:
Error:(11, 41) error: package mypackage.databinding does not exist
Error:(15, 13) error: cannot find symbol class MyActivityBinding
The real error would usually be something like "there's no such attribute android:adapter", or "variable foo doesn't contain property bar" or something like that. But instead of actually showing such errors, it shows the above unhelpful ones, which only tell you that the bindings weren't generated because of some unknown error.

The way I use in Android Studio 4.0:
Select the top level Build: failed item on the Build Output panel
On the right part click Run with --stacktrace. When build is finished select the top Build: failed item again on the left panel. You'll get a databinding error description on the right panel:
In my case it's:
[databinding] {"msg":"Cannot find a getter for \u003ccom.google.android.material.slider.Slider app:value\u003e that accepts parameter type \u0027java.lang.Float\u0027\n\nIf a binding adapter provides the getter, check that the adapter is annotated correctly and that the parameter type matches.","file":"SliderDatabinding\src\main\res\layout\activity_main.xml","pos":[{"line0":14,"col0":8,"line1":21,"col1":41}]}

Clink on Toogle View, under the 'Build' icon will give you details logs

There's no easy way for now. In general, if you face such compilation error and error messages point at missing databinding classes it is usually either bad reference from layout (i.e. you try to access members classes you assigned do not expose - usually happens when you c&p layouts).
Even worse, when you use other code generating libraries (Icepick, Butterknife, etc) then the real culprit can be often in code completely unrelated to binding. So when something like this occur in my code, I usually check Gradle Console view and read it from the end up, ignoring all error messages related to databinding like "missing class" or "package does not exists"

One way of getting the proper error is to run gradle in a terminal with '--info', like:
gradle :app:build --info
That's not exactly integrated into AS, so I can't really accept that answer.

why am I getting the error "error executing aapt "

I'm getting the error executing aapt :return code 1073741819 due to which
.apk file is not generated how to fix this. How to fix if r.java file is missing

It could be a syntax or formatting error in one of the XML files. I just got this error code (it's actually negative), and I checked all the #string references and they were OK. Eventually I figured out it was because of this item in a menu XML file:
<item
android:id="#+id/action_stop"
android:orderInCategory="100"
android:icon="#drawable/ic_action_stop"
android:showAsAction="ifRoom | withText"
android:title="#string/action_stop"/>
However, the spaces around the | apparently aren't valid (XML isn't Java...). Eclipse didn't catch it as an error, but aapt crashed without giving a reason when it saw it. Not particularly graceful handling, by both Eclipse and aapt. Removing the extra spaces made everything work.
The problem with missing entries in strings.xml apparently produces error code -1073741816, not -1073741819. You have to look closely, but the last digit is different.
I'm not saying the problem is definitely some sort of syntax error. But it's another possibility that could be investigated.

I got this error when I had deleted a #string entry that was being referenced in a menu node xml resource.

I had this error and spent time trying to find a solution.
Finally the solution for this error which is: -1073741819 ( check the digits ) was a library was missing from: Properties -> Android -> Library
I just added the missing libraries and got this error resolved.

I got the same problem. What helped me was changing the Project Build Target
Properties->Android->Project Build Target

Run projects from androidviews

I am trying to run the sample project from customviews but facing some problems. i am not able to run any project from the site. Each time getting different type of error. I also searched in the net for the solution but failed to figure out the solution. May be i am facing a general problem for all projects and if i solve one i may be able to solve the others.
In ActionbarPullRequest library i am getting :
Description Resource Path Location Type SmoothProgressBar cannot be resolved to a type DefaultHeaderTransformer.java /library_ActionbarPulltoRefresh/src/uk/co/senab/actionbarpulltorefresh/library line
57 Java Problem
In StackBlurDemo i am getting this error :
Description Resource Path Location Type toggleButtonMode cannot be resolved or is not a field MainActivity.java /StackBlurDemo/src/com/example/stackblurdemo line 30 Java Problem