This is very common issue that your vendor are not able to access the wp store frontend because of access. please follow below step for give them access for submit product page.
Step1)
First go to Dashboard >> Store Frontend >> Membership Tab
Then enable “Enable Woocommerce Vendor” option and save setting.
Step2)
Go to Add New Store or vendor from Product >> Store (vendor)
in this its ask for Store administrator (optional) you need to select user which you want to give access for store publish product.
That’s it.