Datakop
I am not sure we are on the same page.
You say that you opened the table "which looks fine". That's an evaluation, not a description of what you see. We need the description. Best is a screenshot. We're interested in the properties of the table itself, not the data, actually.
You also say, "get a sound and the error message wiggles". We don't need to know about the wiggling, we need to know what the error message says. Again, a screenshot would be best.
I should have stressed how important it is to know if this is a split application, with tables in one accdb and forms in another. I know I asked, but didn't insist on an answer. It's important to diagnosing the problem.
That's also true for the other question. I asked but didn't insist on an answer. It's important. Is the form open at the time you try to change the table to design view?