move not allowed?

Game ID 4603820
Why can't black pawn move C7 to C5?
Pink squares are displayed and the piece remains where it is.
I'm sure it's not en passant. It's not avoiding capture.
What am I missing?
I'm looking forward to finding out.

Game 4603820

You are in check.

Thank you... didn't even see that.
It's too early here in the morning to be playing chess.
I was completly focused on my oppenenys Queen!!!
Thank you