You must have a Nintendo Account linked to your Animal Crossing: Pocket Camp save file in order to transfer it to Animal Crossing: Pocket Camp Complete. To do this, follow the steps below:
Open Animal Crossing: Pocket Camp (NOT Animal Crossing: Pocket Camp Complete).
You'll receive a message that states the service for the game has ended. Click on the "Transferring Save Data" button below.
Tap the "Create/Link Nintendo Account" button.
This will open a browser window. Sign into your Nintendo Account and then tap "Select this account" when asked which account you'd like to link.
If successful, you'll be transferred back to Animal Crossing: Pocket Camp and receive a pop-up message confirming your Nintendo Account has been linked.
With your Nintendo Account linked to Animal Crossing: Pocket Camp, you can now transfer your AC:PC save file over to Animal Crossing: Pocket Camp Complete. Follow the steps below to do so:
Open Animal Crossing: Pocket Camp Complete.
Select the yellow "Continue" button on the main menu.
This will open a browser window. Sign into the Nintendo Account linked to the original AC:PC and then tap "Select this account" when asked which account you'd like to link to Animal Crossing: Pocket Camp Complete.
A pop-up window will appear asking if you'd like to transfer your save data over. Tap "Transfer" to do so.
If successful, you'll get a message stating that your "Save-Data download is complete." You can now load into your Animal Crossing: Pocket Camp save in ACPC Complete.
Note that your save file will now be local to your device so if you delete your Animal Crossing: Pocket Camp Complete app, your save file will also be deleted with it with no way to get it back.