Thank you for your post.
If you can no longer create a field, then something has changed. The challenge is to find what has changed.
Since none of your databases allow you to add a new field, this would generally point to the application. To verify, launch FileMaker Pro and create a new database file. See if you can add a field. If not, then it is either the application or your user account not allowing you to make changes. If you can create fields, then the focus shifts to the database files and possible user account privileges damage.
Keep me posted with any progress.
And there is more than one way to "add a new field" to a FileMaker database. It may be helpful to describe the exact method or methods that you are using to add that new field.
I did start a new database and I could enter the new fields, however if I want back later to add a field it would lock up again. I have reinstalled the software and that has corrected my problem. It is back to working like it should.