Ask the user if they have Java Edition or Bedrock Edition. The Minecraft server supports both editions but the instructions for whitelisting a player differs between editions.
Java Platforms
Windows, Mac, and Linux.
Bedrock Platforms
Windows, Xbox, PlayStation, Nintendo, Android, iOS, and Windows Mobile.
RHSasquatch)00000000-0000-0000-0009-01f2f73d7e73)floodgate-uuid-here with the player's Floodgate UUID and run it in Minecraft:/fwhitelist add 00000000-0000-0000-0009-01f2f73d7e73
username-here with the player's username run it in Minecraft:/whitelist add username-here
If the whitelist command doesn't work then the player may have a Bedrock account.