Tried to check in and select seats for my flight tomorrow at the 24 hour mark. Told:

"We have found your reservation, however we are unable to check you in online. Please proceed to the airport ticket counter and an agent will be happy to assist you. We apologize for the inconvenience."

Why did I get this error message? I am flying from SLC to DCA via DEN and MKE. The segment from MKE to DCA is on Midwest, so I suspect that that is where the problems are. But, if so, why won't the system at least let me select seats for the first two segments, particularly since Frontier and Midwest are both owned by Republic.

This is not giving me warm fuzzies for Frontier/Republic. I typically fly to Washington several times a year and usually go on a Delta nonstop to DCA or BWI. This time nonstop was not an option because the fare cost exceeded (by a mile) the cost differential between nonstop and connecting that my company travel regs permit. So, anyway, I was not happy about flying on Frontier anyway and this issue gives me no reason to change my mind about using Delta as my first option.

According to Seat Counter the flight from DEN to MKE is 0 accross the board. So I am worried that when I check in tomorrow morning at SLC, I will be told, "sorry, no seat for you." Does Frontier have interline agreements with anyone?