Hey TGLP sorry to hear you're having this issue. There have been a few cases of this recently and our developers found that there is currently a slight delay with the inventory of variants coming into Shopify. They are working to fix it right now, but are still in the process of finding a solution.
As a workaround, you can wait. There is no specific time period, but some merchants have reported seeing the inventory after a few hours to a day. Otherwise, you can manually put in inventory as it will allow you to place the order with supplier.
Hope this can help add some insight while we work towards a permanent solution!