The IRS only allows a SS# thru the EFILE system ONCE per tax year ... the child's number has already been used electronically thus YOUR original return claiming the same child MUST be mailed in this year period.
When, if , or how the other parent files the amended return is immaterial to YOUR situation. You can mail in your return NOW ... you do not need to wait for the other parent to do anything. To claim that child on the 2021 return this is your ONLY option.
