Doesn't make a variant as default when import a csv file

Hi,

In your CSV, you should have one column for each characteristic. For each row of variants, you should have the value of the characteristic for that variant in these columns. And to define the default one, you need to enter the value of the characteristic of the default variant in these columns for the main product row of the CSV.
What you describe indicates that you must have left these cells of the main product row under the characteristic columns empty.