I have a funny problem that I cannot upgrade my skills on any character. i only have a button named respec and a text next to the button that my skills are invalid.

Any clue what to do?

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If your rank is vice admiral or your level 50
That's it

This is happening to my lower levels like 38 and 24.

There was a bug recently that allowed people to put points in skills before they were supposed to unlock. Anyone with an invaild skill setup because of that has that button. Click rspec and your skills will be reset and you can reapply them.