NEC ECC: Under Option A contract, if a Contractor omits to price & programme for a specific activity (that has been outlined in the Works Information), is this classified as an ambiguity under Clause 17.1

No - or not at least one you can claim as a commercial advantage.

The activity schedule is not Works Information. It is a payment mechanism during the life of the contract. The risk of errors in the activity schedule remain with the Contractor. Clause 20.1 states the Contractor carries out the works in accordance with the Works Information. The fact you missed something of the programme/activity schedule - there is no where in 60.1 that is a reason that you can claim this as a compensation event.