I went to a port to try trade in some extra goods I had on hand, I set everything to 0 except the good I was selling and got this:

You do not have enough free space to buy the Energy you want.

It seems to be caused by the fact I picked up a power module in a debris field, and was well over my max, but since i'm not trying to sell it (even though the text message says i'm trying to buy it), this shouldn't be an issue.