The message says:
Array of elements of the item to ignore conflicts for, can only contain values of "label" and or "description" and or "sitelink"
However, I can only use values "description" and/or "sitelink" ("labels" were removed with T67990, try it in ApiSandbox (wbmergeitems)).
Additionally, messages apihelp-wbmergeitems-example-3 and apihelp-wbmergeitems-example-4 don't correspond to URLs below at Special:ApiHelp.
Fixed with https://gerrit.wikimedia.org/r/253583 as part of T118322