For 2021 return, it seems that using 99-9999999 generates an error loop and forces file by mail. Using 99-0999999 clears the error loop. The relevant pension income shows up (I think correctly) on Line 5b of the 1040. I am filing Form 8938 to report the UK pension that this relates for my return and reporting there that the relevant income is being reported at line 5(b) of the 1040. I don't want to efile, don't want to mess with Treaty elections. Hopefully this works.
