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I didn't change anything about my description of the problem. Just added information about the site.
The list with the issue was the list with an indexed column. I believe the index was created by SharePoint since I enforced one of the columns to have unique values.
As I stated above, a different list on the same site that is without an indexed column has no issue. It is only the one with an indexed column.
I am the owner of the site. I have full control of the list and the site. However, it is the only site I have control of so I cannot duplicate it on another site.
I made a new list on the same site and duplicated it as much as possible. In doing so I may have found the problem.
The list with the error msg had a lookup to a 'Model Number' list for the PN.
The PN column on the model list had been indexed at one time (to maintain unique values) but it was a straight text entry column. Then I decided to break up the PN and calculate it by concatenating part of the PN together with a unique number on the model list. The calculated column had much of the same information as the old text column but the old one hadn't been maintained once the calculated column was made. But for some reason, I couldn't delete the old column, so I just hid it from the view.
The original list this post is about (with the error message) was still pointing to the old text column, not the new calculated column. That text column had some blank cells.
I changed the lookup to point to the correct calculated column and now I no longer get the error.
The error message would have been more helpful if it gave some info instead of just #ERR.