To be able to vote you need voting power and there are several possible reasons why your voting power may be zero. Some of the most common are:

You didn't have the tokens in your wallet before the proposal "snapshot". You can find the proposal snapshot above your voting power when you are casting a vote.
You haven't enabled governance for your tokens. Some spaces require you to delegate/self-delegate the tokens to enable governance. This can usually be done through the governance website of the organization. Popular space that require governance enabled tokens:
- Optimism - https://app.optimism.io/delegates
- Uniswap - https://app.uniswap.org/#/vote
- ENS - https://claim.ens.domains/delegate-ranking
- Gitcoin - http://decentralized.quadraticlands.com/
Your tokens are on a chain that isn't configured or they are deposited into a lending protocol like Aave.
Each project on Snapshot has it's own governance flow and setup, documentation is a great way to learn about their individual governance process.

snapshot-labs / snapshot