I forgot to enter box 21of my T4AP. What can I do?

You should wait until after you’ve gotten your Notice Of Assessment (NOA), as the CRA will likely correct the error themselves. If they don’t fix it, then you can use REFILE or the CRA’s My Account to make changes to your filed return. This FAQ has more information: https://turbotax.community.intuit.ca/community/tax-return-amendment/help/i-have-filed-my-tax-return-can-i-still-make-changes/00/873608

I am member for over10 year  my email address is [email address removed] Id is John-glenda

I sent john out on friday night at 12 Am confirmation number is 719milu5aea

If you forgot to enter something needed on your income tax return and have already filed your return, you will need to wait for your Notice of Assessment in the mail before you can "Amend" your return if need. Once CRA assesses your return if they have the T-Slip on file they will correct it for you.