Voting, like all RChain transactions, incurs a small charge. (The coop offset this charge with a 100 REV reward for those who registered before July 31, 2020.) Be sure you have enough in your account to cover the max transaction fee.
When you are ready, press Sign and Submit. This will bring up metamask in order to sign your rholang response.
Confirm that your transactions succeeded. It could fail if you don't have any REV.