Geyser is behind the current game version
Applies to Geyser, Paper, Velocity, BungeeCord.
What it looks like in your log
Geyser does not support Bedrock version 1.21.50 (unsupported version)
What causes it
Geyser only tracks the latest Bedrock and Java protocols; after a game update an old build stops accepting joins.
How to fix it
Update Geyser (and Floodgate) to the latest build. They update independently of the server jar.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log