Can I amend my filing now or must I wait until the IRS contacts me? I failed to enter my 2020 IRA distributions as income.

your  paper 1040 could take months to process.

IRS may correct your return for you , since they get the 1099-R also.

Since you mailed the return, I would highly suggest you wait until you hear from the IRS. A paper filed return could take months before they process it.

But if you expect to now owe taxes, you might want to make a payment on the IRS website to minimize interest and penalties.