Can you check your site’s PHP error log right after you get that “This registration step could not be completed. Please refresh the page and try again.” error? There’s likely an error happening at the server level and if logging is activated, you can get that error. If logging isn’t activated, you can follow this guide to use WP_DEBUG mode: https://eventespresso.com/wiki/troubleshooting-checklist/#wpdebug
Also, please consider updating to the current version of Event Espresso 4. Your site uses an older version that’s no longer supported.
With regards to the donation form that’s not something we can help you with because the form isn’t powered by Event Espresso.