RE: Payment approval for purchase invoices
What you're asking to do can be accomplished, but it would require a customization at some level.
The Suggest Vendor Payments reports skips records that contain any text in the On Hold field of a Vendor Ledger Entry. You would need to create a new field in the Vendor Ledger Entry table where your approvers would type something in, and then modify the Suggest Vendor Payments routine so that it looks for any or specific text in that new field if there is also any text in the On Hold field.
Another thought (but it may be too complicated):
Since you are using NAV 2016, you can use the Workflows capability of NAV 2016, which is based on an Event-Condition-Response model. Microsoft has pre-created a handful of Events around tables like the Purchase Header, Sales Header, etc. but it does not include events involving Posted Purchase Invoices (i.e., the Purchase Invoice Header table).
Your partner, or you (if you are a partner or have a dev license) can create your own Events for Workflows in NAV, and then pair them with conditions and responses; and you will likely need to create some approval scenarios around the Purchase Invoice Header table similar to what you have on Purchase Orders and Purchase Invoices (Send Approval Request, Approve, etc.), which allow a record to be restricted, sent for approval, and depending on approval/rejection, determine whether to unrestrict a record for further processing.
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