This is a known bug that we are currently in the process of trying to fix. This ticket is being merged into another ticket so all information on it can be kept in one place to be reviewed.
If this bug happens again and you can provide anymore information on any additional circumstances that made it occur (time spend online before evolving, any held items, any cancelled evolutions in the current game session before this one, etc), just post it below.
To view this new ticket, follow the following link:
https://www.pixelmonmod.com/tracker.php?p=2&t=14241In the meantime, all we can suggest to try is to leave the game, restarting your game and launcher, or restarting the server (if you play/own one) to attempt to fix it. Or default to the /evolve command upon a failed evolution until the bug is fixed.
Thank you for your report.
-Bull