Unable to purchase certain abilities from the guild

[Viridiana]

I checked and my skills (those tier II of stat converts) says Optimal level 40, and my Seasoned Veteran says Optimal level 50. Could it be that we need to be of the correct level in order to purchase? Because for other automatically learnt skills/spells that we obtain from leveling up, they have Optimal level marked with the same level required to learn.

Nope, in this case the level you can obtain them is lower than their optimal level.