First of all the command you would need is pmbootstrap build samsung-a12 --src=<whatever>. Secondly instead of "samsung-a12" you should put the name of the package there. I'm assuming the package you're making isn't called that, and if it was you should rename it as it should be device-samsung-a12 instead.
pmbootstrap is complaining that it can't find the package, as you didn't indicate the right package name. And if it did find the package it wouldn't know what to do with it as you didn't tell pmbootstrap what to actually do (e.g. "build").
But if you want help it's better joining our Matrix channels, in your case specifically #porting:postmarketos.org.