If you typically receive Schedules K-1 from partnerships and you are expecting Schedules K-3 this year, then you will generally need to wait for them before you can file your S corporation return.
If you are expecting a 2021 Schedule K-3 but do not receive it, then you may need to disclose this on your Form 1120-S to protect yourself against the $10,000 penalty. IRS FAQ 16 indicates that a Form 8082, Notice of Inconsistent Treatment, may need to be filed to alert the IRS of the missing Schedule K-3.