Back office staff occasionally and accidentally enter an amount of zero for a donation, which Civi accepts as valid. Is it possible to reject this as an error? Price sets can do this but don't actually prevent the user from not selecting the price set & then entering zero. Using Civi 5.19.4 & Drupal 7.69
I am afraid to say that your use case or business logic is not supported in CiviCRM ATM. You can either raise a request on https://lab.civicrm.org/dev/financial or write an extension and implement hook_civicrm_validateForm hook to throw validation error on screen and not allowing user to submit the form.