Some websites block temporary email domains. This is usually a business or abuse-prevention decision by the website, not a technical failure.
Try Another Domain
If your temporary email app offers multiple domains, generate a fresh address and try again. This may help with simple domain blocks.
Use an Alias
If you need the account long term, an alias is usually the better answer. It protects your primary inbox while still allowing recovery and support messages.
Respect the Service Rules
Do not keep retrying to bypass a service that clearly requires a permanent email. That may violate the site's terms and can lead to account restrictions.
Decide Whether the Signup Is Worth It
If a site demands more identity than you want to provide, it may be better to skip the account entirely.