Facebook Business Page - Cannot Post Inventory

Hello, I'm looking for some assistance. I'm trying to help a car dealership post their car inventory on Facebook Marketplace. They've made me an admin on their page. I helped them post inventory successfully on the owner's account. But when I am signed into my own facebook account, and try to post inventory, it will only let me post inventory under my own personal page. There is no drop down or options to post under the page. I have tried using several different browsers, clearing the cache, using a private window. I cannot get it to work. If anyone please has some idea how to fix this I would really appreciate it.