Trade area rings created with the Dissolve Option parameter set to Split, will result in complex output polygons for each site. Remove Overlap cannot be applied to trade areas created with this parameter value.
Generate trade area rings for each site with Dissolve Option set to Overlap. This value will enable Remove Overlap to be applied across all trade areas.
Alternatively, you can create trade areas for each site with Dissolve Option set to Split. This will disable the Remove Overlap parameter, resulting in the preservation of any overlap between sites' trade areas.