There are 2 easy steps only, go to the following links:
Step 1: Get the API --> https://www.prestashop.com/forums/topic/539319-store-map-error-missing-api-key/page-2?do=findComment&comment=2375038
Step 2: Insert the API --> https://www.prestashop.com/forums/topic/539319-store-map-error-missing-api-key/?p=2360228