Bungie automatically migrated characters who met the requirements back in 2017. Any account or characters made after August or that did not meet the requirements were not and cannot be migrated after.
The requirements were your characters had to be at least level 20 and have each completed the Black Garden mission by end of August 2017 (before the launch of D2 that September).
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I met all the requirements, my characters had all finished all the raids.
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The migration was only for going from D1 to D2 on the same platform. There was no migrating from D1 PS to D2 PC since there was no PC platform prior D2.