Two possible reasons:
1. An update is out: you updated, the server has not yet. We know about the update, but we cannot update instantly: servers have to wait for the Oxide release (the mod that runs plugins). Oxide usually comes out 10–60 minutes after the main update — just wait a bit.
2. You are joining from the test client (Staging Branch). The test version always has a higher protocol than the regular one, so this error will appear constantly. Switch to the regular client.