The maintenance key is the licence key which allows you to update the Bridge 2200 onboard software and so can cause issues if things go wrong.
If your maintenance key is bad for some reason (the most likely cause is that a temporary key was issued during installation which is not usually needed but is from time to time, though there are other possible reasons) and you are trying to add your correct "real" maintenance key but getting an error that a licence already exists, you may need to first remove the old maintenance key.
To be able to do this, you will need to start up the LifeSize Bridge Utility (LBU) and log in as the Support user (this won't work when logged in as the Admin user):
The username and password are "support" and "support" (no quotes, obviously) by default.
Once logged in, navigate to the Maintenance tab, and click on the existing maintenance key as shown, and click the red X (delete) button:
Once the key has been deleted, you will be free to add in the proper maintenance key, which you download from the https://software.lifesize.com/license/index.php site.