The fundamental issue is that mysql will not let you edit a table unless it has a primary key (if you've made it to this post then you likely know that by now).
So solution 1 is: If you have a Primary Key and have not declared it, declare it, click apply and edit the table
But the problem that I ran into was different and I wish this post had been there.
I had created a table to show the many-to-many relationship between two tables. The table consisted of two columns (Table 1 PK and Table 2 PK) but neither of these columns could be the primary key of this join table and I didn't want to create a 3rd column just for a PK.
The solution was to declare BOTH columns as PK! That way the table checks to make sure the combination of col 1 and col 2 are unique AND then I was able to edit the table.
The obvious bonus is that now mySQL will check that I don't enter a duplicate since the combination must always be unique.