I have wasted a whole day trying to get VS 2015 Schema Comparison to work with SQL Server 2017 Express. As per the screenshot, upon selecting a connection, choosing server / database - it connects OK, but upon returning to the 'Select Source Schema' window, the OK button is always disabled; therefore I cannot complete the step to specify the 'Select Source' or 'Select Target' databases to compare:
This only happens for databases on SQL Server 2017; For databases on SQL Server 2016, the OK button is enabled and I can choose a Source and Target to compare.
Download latest version of SSDT for VS 2015; version is v17.4 (in Sep 2018).
NB: The microsoft downloads 'market' is a mess... i found a few links from msdn, technet, etc. make sure you obtain the version above.