M2M Table showing invalidfield, but not sure why

[Aaron Duncan]

I created a M2M table, as I have before, and when attempting to test it by adding various records to it, it says action not authorized, but the ACLs are correct. However, I did notice that this might be the cause. Not sure why it has highlighted the field below, as the style for it says it's not a valid field. I can't edit it, but can anyone tell me why it would be an invalid field?

[Mark Manders · Accepted solution]

Check the field length. I think your issue is there, looking at how it's not finished.

[Aaron Duncan]

Is there no way to modify it? I’m an admin, and even with security admin role on, it still isn’t editable. I did check to make sure that I’m in the correct scope.