Police Special Eucation Vehicles

So I've started to build my PD and upgrade the service but I've hit a roadblock. For a current mission it states that I need to have K-9 unit to finish the call but I'm unable to purchase the K9 unit for some reason. I have units that have been trained for K-9 as well as I have the required amount of PD stations but when I go into the vehicle market there is no button to purchase the vehicle (this is the same for all the other PD vehicles). If anyone knows why I can't buy these vehicles let me know.

Quote from REALAIS

sounds that you dont gave empty slots in your station.

I've been playing this game for over 7 months- smh. Well yeah- You were right- turns out it I was just being forgetful. Thank you!