The IRS accepted my original form 1040 on March 11, 2021. Why can't I file an amended form (1040x) until March 25, 2021?

 The date it should be ready  in the software has been changed to March 18---a few days from now. 

Once your return has been processed you will complete a Form 1040X   --- which is not yet available --- and submit it to amend the mistakes made when you filed your original return. It takes the IRS at least four months to process an amended return.

The Form 1040X you need is under revision and is not ready yet.  The Form 1040X does not become available in the software until March 18 (subject to change).

https://www.irs.gov/forms-pubs/new-information-for-form-1040-x-filers

If you are not willing to wait for the software you can prepare it on a paper form and mail it in.  Either way---expect it to take 4-6 months for the IRS to process your amended return.

You can print out that 1040X form if you want to use it—-and prepare your amended return by hand to mail in instead of waiting to use the software and e-file it.

https://www.irs.gov/pub/irs-pdf/f1040x.pdf