The error means that the formula is on the wrong table. You're basically using the formula on a table different than the one selected in the form settings. When you set up a miniExtensions form, it only works on the table that you set it up for. Please make sure that you are using the correct form formula on the table that it belongs to.

If you're using the legacy form, you would get the error "Record ID does not exist in this table" for different reasons.

You may also get this error if you have selected the wrong form extension for the linked records field. You basically have the linked records field using a form that's linked to a table other than the linked table. When you try to submit the form, the error would say "Record ID belongs to table X, but the field links to table Y". To enable editing linked records (line items), you need to select a form that belongs to that linked table.

Also, you could be getting this error because you're trying to update a record (that doesn’t exist) rather than create one. miniExtensions Forms can only edit records in the table that it was set up for. If you've created the prefill formula using the prefill builder tool at the bottom of the form, then it can only be used to edit 'existing records'.