Hi @Célia Boulbès ,
Welcome to the community!
If you add a label it will show automatically!
Have a look at this documentation: https://help.trello.com/article/797-adding-labels-to-cards
Just clear your browser data and after that log in to your account and I hope it will solve your problem!
Click on the label on the card front to toggle whether the card front labels shows text or not.
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