I must have made an error in my variables (?locationOfDiscovery / ?country) the following version of this code worked fine :

q_list <- c(df3$QID_items) 

qid_list <- c(paste0("wd:",q_list, collapse = " ")) 


query_sparql <- paste0("SELECT 
    ?item ?locationOfDiscovery 
WHERE {  
    VALUES ?item {", qid_list,"}
      OPTIONAL { ?item wdt:P189 ?locationOfDiscovery. }
SERVICE wikibase:label { bd:serviceParam wikibase:language 'en'. }
}")