When using single quotes ' then SAS will not look to find macro references inside. You have to use double quotes for it to happen.
Oh, and you will probably need to add another dot in there.
Example:
%let export=myFile;
%put \C:\Users\Incremental_Reach_Report_&export.xlsx;
*This will then be translated to \C:\Users\Incremental_Reach_Report_myFilexlsx;
*The dot will be translated as the end of the macro variable, and it will be removed when the replacement is happening.;
*You want;
%put \C:\Users\Incremental_Reach_Report_&export..xlsx;
*This will then be translated to \C:\Users\Incremental_Reach_Report_myFile.xlsx;