Seriously, this pops up in game chat every 15 minutes…
Can you just put a message on each mastery tab/section that says something like. “Locked until completing (Main Quest: qusst-name) and assigning 20 points into base class.”
Same message should also be shown the first time you open your passive screen with the first popup “Passive System Introduction”.
A few example solutions. Let me know who to invoice 
Need to click on images to see full image
https://imgur.com/ndP7A7z