This is very common and no reason for concern
Technically,
you would have to wait until your originally filed return is fully processed
and then amend your return to add the missing 1099.
As a practical matter,
however, it is extremely common to omit reporting a Form 1099. You
file before they arrive, they get lost in the mail, get lost in your house or
get tossed along with junk mail.
If
you don't file an amended return to report the 1099, the IRS may
send you an automated Notice
of Proposed Increase in Taxes later this year.
They will calculate the tax due (plus
interest and penalties) and you can send them a check (no amended return necessary).
