Hi Sol007,
According to your description, you are trying to copy and auto-populate another field value.
Generally, you can achieve it through many ways: Business rules, workflow, flow, javascript and plugin.
However, two fields you are using have different type, 'Address1: Country/Region' field is single text, but 'Territory (custom field)' is choice.
The choice field consists of two parts: Label and Value, it displays the label externally but actually stores the value.

When you get value from 'Address1: Country/Region' field, it is text(label), so you need get choice value based on the label firstly, then you can use the value to populate the choice field.
Refer to the following link to get choice value based on the label: