The SEP contribution that you made fore 2023 was reportable as a self-employed retirement deduction on your 2023 tax return, not on your 2024 tax return. It seems that you need to amend your 2023 tax return to include that.
With regard to your 2024 SEP contribution, if you intend to make the contribution after April 15, you should make it before you file your 2024 tax return and report the amount as a self-employed retirement deduction on your 2024 tax return. You can prepare your 2024 before that, including entering the SEP contribution, but don't file the tax return until after you have made the contribution. If you make the contribution after April 15, you must request a filing extension.
