Please provide a reproducible example:
FAQ: What's a reproducible example (reprex) and how do I create one?
If I had to totally guess, I assume that you don't have a column in your data called "Membership", but it is impossible to tell without seeing your data and code!
Further, one wouldn't use quotations within the left hand side of dplyr::mutate() - I'm not sure what the third and fourth lines of your code are trying to achieve.