How do I encrypt an email on my iPhone 10?

Mails sent via iCloud are encrypted.

But if you want to send emails that require a specific password to be opened, then you need to look in the direction of services like ProtonMail or Tutanota. Both services also have apps available in the App Store.

I have the same issue. I can send an encrypted email with a purchased S/MIME certificate but not with my phone even after I import the .p12 file