Although I have used Turbo Tax to file Partnership returns for years, this year it indicates that the required 8453-PE is unsupported and I will be required to mail in the returns. An additional cost I was not planning.

It depends.

Some forms have not been released for filing by some state agencies. 

Some forms have been updated and TurboTax is working on ensuring all of your calculations are processed correctly.

I have attached a link to assist you in determining if this is the reason you are unable to file.  

Federal forms release dates

State forms availability

@DR-V  Some forms may not be available for e-file yet. Please see the TurboTax help topics below for assistance:

When a tax return can't be e-filed no matter what you do

IRS forms availability table for TurboTax individual (personal) tax products

State forms availability table for TurboTax individual (personal) tax products