Some units at level 20 may have still an empty slot for a potential. You can unlock that hidden potential by getting 5 points by completing the following tasks. Then, go to Castlefront Street and talk to Ellet. She will say that she interviewed a rising star from your squad and unlock the hidden potential.
Participate in a battle (1 point)
Kill an enemy unit, heal an ally, repair cover (for example, a sandbag) or tank (2 points)
Capture a base (3 points)
Save a fallen comrade (4 points)
Note: To get five points easily, enter Skirmish 1. Have a unit kill another allied unit. Collect their body to get 4 points for saving them. Select another unit and kill the unit that was killed previously and repeat.