This can occur if the team name has a special character in it. Special characters are not recognized in the search functionality. Please try the following steps, and then attempt to register the participant again.
  1. In Luminate Online select Fundraising > TeamRaiser
  2. Click manage for the TeamRaiser event in question
  3. Select the Team tab
  4. Search for the Team in question
  5. Click Manage for the team
  6. Click Edit Team Details 
  7. Remove or replace the special character in the team name e.g. change & to and 
  8. Click Finish