The easiest way to do this would be to create a single-select field called “country”.
Then, you can paste a return-delimited list of your 200 country names into this field, and Airtable will automatically add all of those values into your single-select options.
Note that Airtable will increase the number of records in your table to accommodate all 200 country names, but you can delete the extraneous records afterwards.
Other ways of doing this:
A text field can be converted to a single-select field, and the text values will be added into your single-select options.
Create a single-select field, and import 200 country records into that field. (Each record would represent one country name.)