Hi Reah ,
Apologies for any inconvenience caused. That's very weird. There can't be more than one main inventory per product. Do you know how they were created? It shouldn't be possible from the User Interface.
Anyway, the only way to fix this is with SQL queries. Please take the ID of the inventory that you don't want to be the Main one and execute this from your hosting provider's database management tool:
UPDATE wp_atum_inventories SET is_main = 0 WHERE id = [your ID]
Please note that you should replace 'wp_' with your database's prefix.
Please tell us if that works.
Best Regards,
José Andrés