I tested this with SSMS 18.9.1 and connected to an Azure SQL DB, opened the View Designer, selected some tables. I then tried the context menu and Add Table was available.
I also tried Viorel's hypothesis, and selected all tables initially, but Add Table was still active in the context menu (as it should be, because the same table may be added twice to a view.) I also tried a new database with nothing in it - the option was still available.
Sometimes funny things happen in SSMS, I would try to close it and restart. I would also make sure that I have the most recent version which you can find at https://learn.microsoft.com/en-us/sql/ssms/download-sql-server-management-studio-ssms.
...Or you can take a radical approach and scrap the View Designer altogether and just create the view from a query window. The View Designer is quite poor and there are many constructs in SQL it does not understand.