eFootball PES 2021 SEASON UPDATE

Launch the MyClub game mode, switch to Contract tab, Scout, Auction search.

Set the grade to 4 stars, scout slot 1 select league Scotland, and for scout slot 2 select nationality Colombia.

There is only one player available to negotiate with, Morelos.

[ORIGINAL SOLUTION - HOWEVER A WHITE BALL VERSION OF KONDOGBIA IS NOW AVAILABLE, SO A SUCCESSFUL GOLD SIGNING NOT 100% GUARANTEED]

Set the grade to 4 stars and scout type nationality/region to Central African Republic.

There is only one player you can negotiate for with this scout, Kondogbia. You are 100% guaranteed to sign this gold rated player using only one scout!

Auction prices vary all the time. I paid 36,000GP for mine.

There will no doubt be other scouts you can use to unlock the achievement but this is the way I did it.

Alternate to the original solution, it would perhaps be a better idea to invest in 2 scouts over the one. Not only is it a lot cheaper, it also allows for a guaranteed gold.

For example, 4* Iran and 4* Zenit - Russian Premier League; look for the bear, (combined price as of now, less than 30k) will guarantee you Azmoun.
#Other options for Zenit are: 4* Croatia (Lovren), 4* Ukraine (Rakitskiy) and 4* Colombia (Barrios).

Similarly, if you know of a team (or league) with only golds of a certain nationality, those would work as well. It saves you from grossly overspending on a non-guaranteed Central African Republic player (one gold, one white ball).
Another example would be a 4* Burkina Faso scout combined with a 4* Bayer Leverkusen scout to find Edmond Tapsoba. (Thank you, IncredibleKoosh, for finding this one).

Edit: AssassinDMG has found another good, very cheap option - Togolese Getafe defender, Djene (Spanish 1st Division).