This Seeking Tech guide will explain how you can transfer your save file from the Prologue Demo of Metaphor ReFantazio to the full version.
For the first step, make sure you are playing on the same account and platform. Otherwise, the transfer may not work.
Once you are ready, open the full version. During the boot up sequence, select the Yes option when you see a system message asking if you want to fetch the save data from the demo version.
Your save data from the demo should now appear in the Load Game section of the full version.