Unable to file form 10IE to opt-out the option after due date

File revised  ITR under new scheme only, it will get processed under old scheme, as the form was filed late.

As you cannot file ITR under old regime, better to file it under new regime, if accepted by system.

Processing will be under old regime only, because the form 10IE will be rejected due to late filing.

If there is demand, you will be required to pay at that time.