Unable to update iPhone SE 2020 to iOS 18

All iPhones that support iOS 17 also support iOS 18, so you are able to update.

If you are having trouble updating, please review:

If your iPhone or iPad won't update - Apple Support

Otherwise please elaborate as your question is unclear.

It sounds like you have an iPhone SE (2nd Gen), that was introduced in early 2020. Can you confirm? If that's the case, you can upgrade to iOS 18. If it's earlier than that, unfortunately the device is not able to be upgraded to iOS 18.

See this user tip.

Writing an effective Apple Support Communities question