@AlexeyKhimich I see! By default, Shopify has certain order status flows that are enforced to ensure the integrity of the data and the financials of the store. It’s possible that the order status “Pending” is not configured to allow for manual marking as “Paid” in the admin panel.
It may be possible to use a Shopify app from the app store that allows you to customize your order statuses and the transitions between them.
I will do some digging and let you know!