Installed Calendar but can't add events to it.

[alingrecea]

I just installed Calendar and Other plugins but When I go to view the calendar I can see it but I cannot add events to it.

[LordGrey]

I had the same issue when upgrading from RoundCube7 to 8.
Check your SQL tables, the events ones should have a 'URL' column.  If it's missing, you need to run the SQL file from the calendar plugin folder.  If that fails, try deleting the events tables and try it again.