According to the note on https://moodle.org/plugins/mod_flashcard it seems that there are now two forks of the flashcard module:
So you need to check which one you are using.
It seems like the tmuras version is no longer supported, so if that't the one you're using you have a problem...
Once you've established your version, download the code from the corresponding github.com repository and REPLACE the flashcard folder on your Moodle server with the one you downloaded (NOTE: REPLACE the folder, don't OVERWRITE it with the new one).
Then go to the Notifications page in your Moodle Site Admin block and let the plugin be upgraded.