Why is Auto-Number greyed out?

I understand auto-number can't be selected for a primary column. However, even on other columns when I select Auto-Number/System as the column type, the first sub-type, Auto-number, is greyed out. It doesn't matter if i create a new column at end of the sheet, it is still greyed out. I can't find any explanation for this. Any ideas? On other sheets, it works. I am an owner/admin for the sheets in question.


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