After updating my Android SDK I stumbled upon this very problem and I tried many ways without success. What was most irritating to me when searching for a fix, were the lots of answers suggesting to change the CompileSdkVersion to a certain number while obviously this number changes with time, so here's what I did instead.
I created a new project and ran it on the emulator to make sure it's working, then checked its "\android\app\build.gradle" file and copied the numeric value of CompileSdkVersion and pasted into the same file in my other project that could not be built properly anymore. Now my problem's gone. Hope that helps.