On free plans you can't, on business/enterprise plans you can.
Edit: Gemini says you could make a custom worker and do a 5XX redirect that way, but I haven't tried it. Does make sense though.
the custom error page is configurable at a domain (zone) level
which sometimes gets annoying because branding for subdomains could be different.
https://developers.cloudflare.com/rules/custom-errors/edit-e...
This works for errors at the origin or in a Worker, but won't for errors at the edge, because those are in front of Workers.