If you just installed the latest preview of SSMS 18.0 you might have realized that the Database Diagrams are gone. The feature is deprecated.
Download the preview here:
Scroll down to the SSMS 18.0 (Preview) and try it out, there is also improvements.
So, what to do? Remember there is a bunch of INFORMATION_SCHEMA-views you can use.
This is an example how you can do it, the example is not perfect but help you out a bit.