Solved
Unable to upgrade skills of certain units
Some of the unit's skills can't be upgraded, can look into this ???
If you're attempting to upgrade your skill to level 4, I'm fairly certain it requires the Mk-III item which you might not have enough of.
If it's not showing up in the lower right side as a required item, it's because of a display bug that's been reported multiple times already (and is hopefully being fixed by the devs).
