I cannot class up. What should I do?
In order to class up (increase your max level), you will need to unlock Boost Cores.
In order to unlock Boost Cores, in addition to Souls and Zeni, you will need to unlock a certain amount of Boost Panels.
You can check for more details on Soul Boosts using the following method:
Menu > Other > Help > Soul Boost > Soul Boost
Menu > Other > Help > Soul Boost > Boost Panels
Menu > Other > Help > Soul Boost > Boost Cores