Hello,
Debian 10 Buster ships PHP 7.3 but ownCloud (no matter the version) doesn’t support it yet.
I wouldn’t suggest you to downgrade Debian, but with PHP FPM you can run several PHP versions. You could install an “older” PHP version, like 7.2 which is supported, and configure your web server for ownCloud to go with PHP7.2. You might try this, I wouldn’t advise it fully since I never followed this tutorial, and I didn’t do test on Debian 10 yet. But it’s for you to have hints on how to save it 