Does anyone know why it says Tax Year 2021 transmissions are not available?

It could be that a form in your return is not yet ready for efiling, most commonly a state form.   Are you filing a North Carolina return by any chance?

Some NC forms (including the main Form D-400) are not yet finalized and available for efiling.   Projected availability date is 2/24/2022, according to the availability tables:

State Forms Availability Table:
https://ttlc.intuit.com/community/forms/help/state-forms-availability-table-for-turbotax-individual-...

If you wish, you can efile the Federal return now (if all your Fed forms are available) and efile the NC state return later.  To do that, you would need to temporarily change the filing method for the state return and tell TurboTax that you intend to "mail" the state return.  Then when the state forms are available you can switch the method back to efile.

If you need to check on Federal forms:

TurboTax IRS Federal Forms Availability Table:
https://ttlc.intuit.com/community/forms/help/irs-forms-availability-table-for-turbotax-individual-pe...