Ah yes, I think you're exactly right, Alex. I should have read the message more closely:
Lingvist Free daily limit reached
You have reached your new word limit for the day and practiced all your words. Come back later when our algorithm has generated more words for you to practice.
Alternatively, you can upgrade to Lingvist Unlimited and access more new words, along with a whole host of other great benefits.
I was just confused because I knew there were cards that I had answered incorrectly, and I didn't realize that the algorithm will only let you practice them again after a set amount of time.