When putting a fishing rod in an enchantment table, usually most if not all of the enchantment options do not display an enchantment when you hover the mouse over them, and these blank ones cannot be selected. Through later testing and experience I found out the fishing rods are trying to display all the same enchants that a book does, and then anything that's not unbreaking, luck of the sea, or lure just does not display and cannot be selected. The earliest version I experienced this in was 1.16.3 but it continued in 1.16.5 and 21w14a, so it may have been a thing ever since 1.8, when enchanting got changed to the modern system.