Hello Jaey, 

Kindly make sure that you are ordering "Stockable" product. 

because inventory is not managed for service type product that's why "Receice Products" button is not visible. 

Thanks

Anisha Bahukhandi

Hi,

Please check,

1)  Make sure the quantity added in purchase order line is not zero

2) check purchase->settings->purchase order approval is enabled

Thanks

Aswini iWesabe