You need to update your datapack, at least the file:
gameserver\data\scripts\handlers\admincommandhandlers\AdminSkill.java
Or you can change by your own:
if (sk == null || !sk.getCanLearn(player.getClassId()))
And make it like that :
if (sk == null)
Of course , the easiest way is to update datapack.
Be carefull, It is better to update both datapack and core at the same time because sometimes, problems like this appear If you don't have them syncronized.
Compile the DP just make a zip file to prepare files to be uploaded to your server, it is your choice to compile it or not (you may update a single file).
Core is need to be compile always because the source code become machine code, eclipse generate a .jar file.
Hope it helps you.
Good luck.