This seems like a fundamental question, but I have searched and not found anything similar. Please excuse me if I have missed it.
Having successfully used 'PayPal - website payments standard' as payment processor in CiviCRM/Drupal 7 for many years, I am having difficulty setting it up in CiviCRM/Backdrop instance. I wonder if someone might understand what is happening here: At very first setup instance, selecting anything but 'PayPal - website payments pro', including Dummy Payment Processor, simply closes the setup window and returns me to the (empty) Payment processor list. PayPal - Website Payments Pro allows me to enter details and save that option (though client does not have a Pro account).
Environment: Backdrop CMS 1.24.0 MySQL Database: MySQL, MariaDB, or equivalent version 5.5.5-10.4.20-MariaDB-1:10.4.20+maria~buster-log PHP Version: 7.4.33 Web server: Apache/2.4.56 (Debian) CiviCRM 5.58.1