A participant may experience this issue if the Participant Type they registered as isn't enabled for fundraising.
Note: If you're not sure which participation type they chose, this is visible on their Registration Details.
To check if this is the case:
  1. Go to Fundraising > TeamRaiser
  2. Click Edit next to the TeamRaiser
  3. Go to Manage Participation Types
  4. Click Edit next to the Participation Type the participant registered as
  5. Check if "Yes, this participation type involves raising money" under "Fundraising Enabled" is unmarked. If so, a participant of this type would not have access to the Participant Center.

You can enable fundraising and the Participant Center for all participants of that Participation Type by marking "Yes, this participation type involves raising money". You can also modify the participant's participation type to a different one after registration instead.