Review your relationship settings again.
You may also want to check the settings on the edit box too.
Also check the settings for the field in the table to ensure that they are not locked at entry.
I've double checked everything you listed, and there's nothing to suggest I would be getting this error, yet it still happens every time I try to add a record from the portal.
I have also found that I am able to add records directly in the table for tests, and I can edit older entries in the portal. So adding new records from the portal is all that is broken.
You appear to have this relationship for your portal: (You probably have different field names, substitute your own for mine.)
Compound::CompoundID = Tests::CompoundID
Check the field you've used for Tests::CompoundID
Is it a caculation field?
If so, that's why you are getting this error. Change it to a data field with the same type as Compound::CompoundID.
That worked! Thanks for your help.