Hi! Thanks for coming over here instead of the wiki. It’ll make this process easier.
First lets start with a clean 12 system. Fully upgraded.
Then run:
amportal a m
update admin set value = '13.0.0alpha1' where variable = 'version';
exit
Now make sure that the value in the admin table for version IS set to 13.0.0alpha1. Post here on the results but don’t move forward yet, please.