Why does “This gear is fully enhanced” display, and stop me from enhancing a character’s unique weapon?

SSR unique weapons are different from normal pieces of gear, and you can only enhance them up to Gear Lv 240.
When you enhance a unique weapon to Gear Lv 240, you can awaken them to UR.

Unique weapons UR or higher are the same as normal pieces of gear in that they can be enhanced up to the same Lv of the character equipping them.

*It is not necessary to raise a unique weapon’s Upgrade Lv in order to enhance/awaken it.