Is the deadline to file back taxes for 2021 on April 15 or April 18?

Midnight on April 15, 2025----your envelope has to be postmarked no later than midnight 4-15-2025 or the refund is forfeited.  A 2021 return can only be filed by mail; it cannot be e-filed.

You should be able to receive a refund.

This IRS Taxpayer Advocate Service website states that the 2021 tax return filing due date was April 18, 2022.  

Retain a copy of this page should there be an issue with your tax return at a later time.

It is April 18th because the filing date was April 18th 2022!!! 3 years from that is April 18th 2025!