I created Anki cards on Desktop version, sent them to my phone, and tried to open/upload them to AnkiDroid app. An error popped out:
500: Your collection needs to be upgraded to the v3 scheduler.
A few months ago I did the same thing but had no trouble uploading.
What does the error mean? Which version am I supposed to upgrade? The Desktop one? (It’s 2.1.8, but I had no trouble uploading from it until recently). The Phone one? (I don’t see the upgrade option).
Are you doing this with an export from Desktop and an import to AnkiDroid instead? Is there a reason you’re not just syncing your collection between devices through AnkiWeb?
It sounds like you’re using a current version on Android (you didn’t say what version you’re using, but if you downloaded from the Play Store and it is auto-updated, it’s at least 2.18) and a very old version on Desktop. The v2 scheduler is no longer supported, so your collection will be converted to the v3 scheduler. When you got that message, was there a button to convert your collection? Click that.
Yes, please do update your Desktop install. There are a LOT of changes in the 5 years since 2.1.8 came out. If there are reasons (like add-ons) that held you back from updating before, you might not want to update to the current version, so please check any essential add-ons for compatibility before you do.