James... how can I help you if I can't see the expression? The error message alone doesn't tell me enough.

Sorry about that, here is what I have. Both forms "Record Source" is using SQL.

I have a suspicion that it's not a problem with your subform, but that it's a calculation that is crazy somewhere. If you remove the subform, do both forms open individually?

Yes, they do individually.

Looks like you're using an SQL statement for the Record Source properties of your forms. Replace that with a single table. See my screenshot:

See how yours says ? Get rid of that. Go with tables or SIMPLE queries only. If you need to look up other fields, use DLookup on that form.

Yeah, that's WAY too complicated. You'll need to scale that down to a SINGLE table or a SIMPLE query. DLookups in there are a no-no. You MIGHT be able to put a lot of that into a Query and then use that as the recordsource, but as it is now, it's too complicated.

Why are you using a DLOOKUP in a Query, just JOIN the tables.

I was trying not to have so many Tables in the Query. I should have used DLookup in the Form fields.

Most of the "DLookups" are looking up Data from the Helper Table for 'Status, Types, or Unit of Measurement ". To convert the Number Value stored in the Child Table.

You can have as many Tables in a Query as necessary. That's it's job.
Granted you may get the Not Updateable.

I would say there really isn't a reason you should ever have a DLookup in your query SQL statement. Join to the table where the lookup is. But yes, I'd prefer them in the form.

Thank You Guys again for your help on this Problem.