Why I can't use la mienne? Can someone explain to me?
"J'ai acheté les miennes dans ce magasin." is accepted, so I find it hard to believe that "la mienne" is not. Are you sure there was no typo?
'j'ai acheté la mienne dans ce magasin' was accepted for me. So, I assume that you made a mistake in your answer.
Is this wrong?: "J'ai acheté la mienne dans ce magasin-ci."
There is no emphasis in the English sentence to justify the emphatic suffix "-ci".