Could you ask this with a minimal REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.
If you can use a subset of your data, it will be easier to test. merge is used with " in the help page, and it is as I used it. You can also post your previous error to see if it is related. Also, you have in your code
#rename key column to be a shared name CDS_CODE
names(frpm) <- c("CDS_CODE"-)
why the - ? is it working ?