I missed to add another Federal Tax withheld. I already submitted my Tax file through eFile. Can i Still Correct It?

unless your return is rejected, you must wait until the original return is processed - you receive your refund or the IRS cashes the payment before you can file an amended return

I am in the same situation, my 2020 return is accepted but not getting my refund yet, shall i able to file an amendment? thanks a million! 

WAIT ... the IRS usually catches this error.   Amend later only if they do not catch it... you have 3 years to amend.