No Ship From address showing when I try to create my first FBA shipment

I’m trying to create my first FBA shipment to Amazon warehouse but just can’t figure out why there is no ship from address showing on the Send To Amazon page. If I try to add an address it doesn’t let me save it and the ship from address is already saved in my account as a Default Shipping Address under General Shipping Settings. Any ideas please? This is driving me mad and costing me far too much valuable time!