How can I sell pallid sturgeon?

That sounds like a glitch - you should be able to sell the fish you've caught.

Since you were playing in co-op it's possible that the game bugged and didn't register your friend's catch. This is more likely if he isn't the host since that may generate a more tenuous internet connection. If your friend were the host then it would be less likely, but not improbable.

Your friend may have quick sold the all the items in their inventory and not realized that also sold the fish. The quick sell button from the shop's menu includes everything except for collectibles and items used in crafting. Fish falls into neither category so if your friend wasn't paying attention he may have unwittingly sold their catch.

In this situation I'd recommend catching another fish and seeing if the problem persists. I would expect this to be a one-time occurrence.