I had the same experience---I entered "Foreign US" on form 926 for foreign corporation and although Lacerte did not reject it-It was rejected by IRS several times.

In frustration  I just entered 11-1111111   in the EIN Field --and it did not generate any diagnostic by Lacerte and the return was accepted by IRS.   I'm not recommending this as I don't think it's the correct solution to what appears to be a programming deficiency on part of IRS.   Nevertheless, it did work as I was unable to find any other solution

Jerry