Technical Issue when Submitting Sales Order

We’re using Rest APIs to create Sales Order, and I need to submit this Sales order as “Completed” using Rest APIs also, Kindly find below the steps I did to make it Completed but it fails,
1- Created a delivery note
2- Created a Payment Entry (Using Cash method)
3- Created a Sales Invoice

But still status of sales order is “To Deliver and Bill”, hot to submit it and to be “Completed” Through REST APIs ? is there any missing apis ?

Also If I tried to change the Sales Order status directly it throws an error “Not allowed to change Status after submission”.

Kindly advice.