@angeloaliano1 you have already asked this question (twice) and gotten a reply.
In order to get it to work, you will have to implement the function in CPLEX.jl.
If you’re interested, post on the issue (MOI: implement RelativeGap · Issue #228 · jump-dev/CPLEX.jl · GitHub) and we can point you in the right direction for how to make the fix.