Well. If you have that issue, the only way to fix it is to restart the game.
Since the language you used is possibly English, this kind of error is pretty rare occasionally in both Male and Female gender.
The last person who got that error was
@emmauss last May.
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But if you change the language from English to other language like Korean or Japanese, you will get that error ALL THE TIME.
The only way to heal your Pokemon is either buy a lot of Potions, go back to Littleroot Town, or let someone heal for you like May or Steven.