If the products are imported, then at the bottom of the settings there should be a report (how many added, how many updated), you do not have it and the products are not loaded.
The checkbox "Update by supplier code..." does not work because you think the action will not search for products only by this code.
For processing, it is necessary that the product be found in the system, for this you need to specify the external id or article, or search by name, and only after that the system checks the supplier's code and updates the price.
Needs improvement here
The second point here
https://crm-onebox.com/ru/support/suppliers-and-prices/3467-otsenit-dorabotku-de... just about this