#collect is a method you find on Enumerable objects; for example, Arrays. It is a synonym for #map, and lets you apply a block to each element of the collection:
[1, 2, 3, 4].collect{|x| x * 2} # => [2, 4, 6, 8]
You can't send #collect to an integer (which is what you're doing here). It's undefined, and doesn't make any sense anyway.
What are you trying to do with this line of code? My guess is that you're trying to get the full_name attribute of the user that the effort belongs to.
If so, remember that the association gives you a method to get the full User object, not just the User's ID:
effort.user
Once you have that, you can call the #full_name method directly: