undefined method `full_name' for nil:NilClass

You can go into the rails console and check record(s) you're trying to get first_name and last_name from and make sure they aren't nil. An example of this inside the rails console would be, if looking for instance at a users record:

user = User.first  # This will set user equal to the first user in your records
user.first_name # Checking whether that user has a first_name field and what it is
user.last_name # Same as above but with the last name
user.first_name = "The users first name" #This would set the users first name if it's empty and the field exist, can be done with last name as well
user.save #This saves the user to the DB 

Whatever you are trying to pass into or call 'full_name' on is nil when you are doing it. An example of what this would look like is:

def full_name 
  first_name + " " + last_name
end

If you never set what first_name or last_name are, calling the full_name method will give you the same error, you may also want to check your database and make sure what you are calling have a last_name and first_name or those methods will give a similar error.

tl:dr: 'for nil:NilClass' usually means you're trying to do whatever you're doing on nothing.