This is a common mistake for newer players, I even had the same issue myself when I started so don't worry about it. When you do /vote which redirects you to https://www.mc-complex.com/forums/pixelmon/vote/ and see all 8 voting links you'd want to click the little icons next to the votes which will directly lead you to the voting page which does not give the error that you stated. Below is a screenshot on what I mean precisely.