I suggest you contact Sky for assistance, as the message from the app makes no sense.
An explanation of the transfer process for anyone that doesn’t already know
Although archaic, the Mobile Number Portability (MNP) system is intended to provide a near-seamless migration of your number from one provider to another (i.e. giffgaff > Sky), with the only downtime being the time it takes the technician to process the transfer.
As the name suggests, a PAC (Porting Authorisation Code) is a code obtained from the provider that has your number currently, authorising the transfer of the number to your new provider, and can only be used if the number you want to transfer is active with your current provider (you cannot transfer a number that has been suspended (e.g. reported lost or stolen) or has been cancelled/terminated). After the porting request is accepted by your new provider, the number keeps working on your old SIM until the day of your transfer. It will then stop working on your old SIM and start working on your new providers’ SIM later the same day.
A PAC is valid for 30 days. If not used (given to, and accepted by, a different provider) within that time, the PAC expires and your number stays where it is.