Problem :
Error:Could not find com.android.support.constraint:constraint-layout:1.0.0-alpha2.
Required by:
myapp:app:unspecified
For me it was a completely different issue. When I installed constraint dependancy in SDK tools, the tools somehow wrote them into the wrong directory. That is
/home/${USER}/Android/Sdk/extras/+m2repository+/com/.../constraint
instead of
/home/${USER}/Android/Sdk/extras/+android+/+m2repository+/com/.../constraint
Remedy:
Just copy the 1.0.0-alpha*
directories into the latter path