This may be the bot responding to a command with an error that includes "Not found in cache". This is typically because only slash commands are authorized on the server, not the bot user as well. To fix this reinvite the bot with the offical invite link.
There are two possible reasons for this. First the user's discord app may be out of date, and they will need to update.
The other reason is that slash command permissions have been set for that command that hide the command for that user. See the discord docs for more info
This includes context menu commands.