Amending a tax return means filing a special form, Form 1040-X, to make changes to a tax return that has already been filed and accepted. If your tax return was rejected it has not been filed, so you do not amend it. You just make the corrections and try again to file it. You can still e-file.
An IP PIN is required for anyone who has had an IP PIN assigned by the IRS. The IND-996 reject code could be for any dependent on your tax return. One of your dependents was given an IP PIN by the IRS. You have to either find that dependent's IP PIN or remove the dependent from your tax return. If your grown son obtained an IP PIN, and he has not told you what it is, deleting him as a dependent should eliminate the rejection.
See the following TurboTax FAQ for more information about reject code IND-996.
E-file reject IND-996