Why don't I see the Details button for my Service Order Confirmation?

Your Customer must have the entitlement Allow Supplier to enter service details for OC enabled for their SAP Business Network Buyer account in order for you see the Details Button during Confirming Service Purchase Order

Please ask your customer to request SAP Ariba Support enable this Entitlement for them.

Purchasing > Purchase Orders > Order Confirmation
SAP Business Network for Procurement & Supply Chain > Transaction Documents
SAP Business Network for Procurement & Supply Chain > Transaction Documents > Transaction (Purchase Orders, Ship Notices & Goods Receipts)