Check for the NPM license that you are using. I don't think its an unlimited license.
If its the older version of NPM, login into your SolarWinds Server, search for license manager from windows start and check what license are you using.
If its the new version of NPM, login into your console, goto settings -> allsettings -> License Manager
when i say unlimited license it should be NPM SLX.
Please refer the attached screen shot
This error is returned when you've reached the maximum number of interfaces, nodes, or volumes has been reached. You will need to remove/delete some of those elements currently being monitored before you can add any new ones. Alternatively, you can upgrade your license by contacting your sales rep. firstname.lastname@example.org.