I don’t use Plesk. When I tried it many years ago, it made my life miserable because it was so hard to change everything - Therefore, I can’t offer you much help as I obviously don’t know how to use it properly myself.
However, here are some responses to your post:
- If you installed Magento 2.3.5 from scratch yourself, then the cronjobs should have been installed as part of the installation script automatically.
- Explanation for
>/dev/null 2>&1can be found here. More information on setting up cronjobs can be found in the Official Magento Documentation. - You error makes me think that the Database Credentials are either a) incorrect b) MySQL port is blocked c) MySQL has stopped (which are all unrelated to cron)
As I mentioned, your Plesk environment has too many unknowns for me to narrow down your issues as it introduces variables that I may not have considered.