Check to see what the Business Rules are for Events and Fee's in the Raiser's Edge program. 

1. Click on Config > Business Rules > Events
2. Under "When adding registration fees to participants..." select "Do not automatically create nor prompt to create a gift for registration fees".
3. Sign out of the Raiser's Edge program by going to File > Exit and Sign Out
4. Have the user sign out of the Event Mobile App and log back in. They should now be able to add the registration fee without the error message.

Also,

Make sure the user has rights to view and edit Registration Fee's in the program.

1. Click on Admin > Security > open up the group the user is in.
2. Click on Records > Options
3. Select Participants (Events) and under Event Privileges make sure View and Edit are checked for Registration Fees