Watching tutorial video in here: https://www.youtube.com/watch?v=vDuxVmNiwrc&list=PLZSqbv2i8LOBCosw-vNCjyhmJ5BmrmCqA&index=36​
Go to Admin > All products > Choose products
Click to 'Add variants'
Fill variants information and save
Result:
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From The4
Hope you success!!