When trying to build OpenStreetMapView from git://github.com/osmdroid/osmdroid, I get this error:
failed to find target with hash string android-23: D:\\User
Had the same issue with another number, this worked for me:
Click the error message at top "Gradle project sync failed" where the text says ´Open message view´
In the "Message Gradle Sync" window on the bottom left corner, click the provided solution "Install missing ... "
Repeat 1 and 2 if necessary
23:08 Gradle sync failed: Failed to find target with hash string 'android-26' in: C:\Users\vik\AppData\Local\Android\Sdk
Android SDK providing a solution in the bottom left corner