Is this estate using a calendar tax year? If so, this Form 1099-R must be reported on the 2021 Form 1099-R. If this income was required to be distributed in 2021, even though it has not been distributed it's still treated as Tier 1 distributable net income and goes on the 2021 Schedules K-1. If this income is not required to be distributed and was not distributed by the end of 2021, the tax liability is retained by the estate and does not pass through to estate beneficiaries. Note, however, that the executor can elect to treat a distribution made in within the first 65 days of the estate's subsequent tax year as having been distributed on the last day of the previous tax year; we are now past that 65-day period for a 2021 calendar tax year.
If the estate has not previously filed Form 1041, the estate has not yet established the tax year as being calendar or fiscal. It may then be possible to use a fiscal tax year beginning in 2021 and ending in 2022 which would make any income passed through as being taxable on the beneficiaries' 2022 individual tax returns. Items passed through are reportable on the individual's tax return for the year that contains the end date of the estate's tax year.
