I suspect your issue is that you are trying to copy and paste file content into the Setup UI for Apex classes, one class at a time. This would be difficult to do as you must start at the bottom of the class dependency tree and work your way up.
Instead, you need to have all the code (all the relevant Apex class files) on your local file system and use one ...
This type of error(Cannot assign to read-only property) generally occurs on update of public property @api decorator variable value.
As I see from your code you are trying to update pfList property, Check if this public property(@api), If So change into @track or clone the pfList into a new variable.
let tempPickList = this.pfList
You don’t need to deactivate the picklist MN rather change the value of MN to Mentor, just editing the picklist.
Otherwise, from translation workbench change the translation of MN for English language to Mentor.
It will work.
Stumbled up on this old thread. If somebody needs to set up a dynamic Dropdown menu with a collection today, then use this Flow component: