My products keep getting disapproved because of "missing shipping" although I have added/imported shipping service for the country in GMC?

Other feed apps, including the Shopify Google app are known to override shipping settings in GMC regularly. So entries that are manually added or imported from other apps are overwritten.

It is recommended to use just one feed app. However, if you can’t remove the other app for some reasons, you can specify shipping settings at the feed level. You can do that in the Shipping tab of the feed editor, which will override shipping settings at the account level.