Tried to file an extension.

New corporation in 2021. EIN confirm received stating that the return is due April 15.  Therefore, the IRS assumes it will be a C.  But the client has not yet filed 2553 so the IRS doesn't know it will be an S.

Extension request was rejected with the message: "The IRS believes that the entity type of the client does not match the entity type in the electronically filed extension. You must contact the IRS to resolve the error."

Phone call attempted but the IRS is extremely busy.

This situation must be somewhat regular.  I'll file a paper 7004 unless someone has a solution for me.