When you hit level 50, most players are close to or in the final mission of the game. At this point, you can't add skill points like you could earlier. The skill tree gets buggy, and if you refund any or all of your points, you won't be able to assign them again. If that happens, you'll need to load an old save to fix it. It’s super frustrating.
Just wanted to confirm this. My rook recently hit lvl 50 and decided to tweak my build for the next area to take advantage of changes in resistances and am unable to do so because of this bug. If you fully respec your tree you are f'd and will need to use a prior save, but sometimes if you haven't refunded all your points you can fiddle going in/out of the skill areas to get it into a state that will allow changes... it doesnt always work, but sometimes it does.
I think it caps at 50 as part of the game. Dragon age always employs capping skills. You can probably reassign your skill points though.
