First-time farmer with basic question

I believe I understand the basics of farming....have 3 fields and wanted to start with cotton. I have prepped the field with 5 Humus, and purchased 2 stacks of 25 cotton that is 15% pure from a player stall. I couldnt plant it on my fields.
I just went to Boston and purchased one stack of 50 seeds of cerial and was able to plant that.
So, why couldnt I plant the 50 cotton (in 2 stacks of 25) from the player stall?
I just went to Boston and purchased one stack of 50 seeds of cerial and was able to plant that.
So, why couldnt I plant the 50 cotton (in 2 stacks of 25) from the player stall?