This usualy happens in winter when the temperature is freezing, just wait until it warms up a bit an you shoul be able to plant again. The odd thing is the greenhouse though that should have worked that maybe a bug that needs to be looked into as I aggree freezing or no if its inside it should not matter so that needs to be looked into I belive...
You could also try using the weather machine to raise the temperature. (Dr. June's Weather Control Device)
I figured out I can plant things if I drag them into the world first, but that plant option straight from inventory never works so seems a bit pointless to me, my game also went into spring by the time I figured this out so will have to see if it still works come winter again
