In order to vote for a proposal (off-chain or on-chain), you would need to have a non-zero voting power. Your voting power is calculated for every proposal at the time a proposal goes live. Therefore, if your external wallet received your BTRST tokens after the start date of the proposal (or if you did not “Unlock voting” and self-delegated your votes in the on-chain portal as per the on-chain voting setup steps), your voting power for that proposal is calculated as zero and you would not be able to vote for it. However, you will be able to vote for any other proposals that go live after you received your BTRSTs.
If you did receive your tokens in your external wallet before the proposal went live, it might be a small caching problem on Snapshot. To solve it, we recommend you refresh the page several times and/or clear your cookies for that page (here is an article showing you how).