Please ignore that, it seems it's due to some technical issue you received such notification or trophy, as for the reward and trophy itself, you need to have 5000 posts or messages posted to this forum which isn't the case here.
You can ignore it and you can send me the screenshot of the notification or a full screenshot of when did you receive this error and stuff, so I can look into this and work on it
P,S I checked your account and I don't
see a trophy assigned to your account, could be a glitch.