I got a schedule K-1 that has box 16 Schedule K-3 is attached checked. However, it also says the schedule K-3 will be made available by the end of June 2023. What to do?

The K-3 is used to report foreign income.  Many are receiving K-1s with box 16 marked when it is known they will not receive a K-3 because there is no foreign income.  If you know you will not receive any foreign income, you can unmark box 16 and continue with the filing of your return.

If you are expecting to receive a K-3 with foreign income, you will need to file for an extension and file the return when the K-3 is received.  Remember that if you file for an extension, the extension is only for filing the return and not for payments due.  If you expect an amount to be due, payment must be made by the filing deadline.