Can't import my skin into Minecraft

Whenever I import my custom skin into the game, it keeps popping up "that's not a Minecraft skin, silly" despite choosing the right skin format.
Steps to Reproduce:
1. Go to dressing room
2. Choose classic skin
3. Press owned skin and choose new skin

Observed Results:
It keeps popping up " that's not a Minecraft skin, silly"

Expected Results:
It should have imported my skin into the game