You can see from this screenshot that the key is added. But, when I go to write a new email the key does not save in the encryption settings panel, but the keys are in the OpenPGP key manager. Also why is all the OpenPGP settings greyed out?

In OpenPGP key manager, if you click one of your own keys, what does it say? Does it say it is of type personal key? Did you select the choice "accept as a personal key"?

Account settings only lists keys that are accepted and contain a user ID with an email address that matches the email account.

Composing OpenPGP message doesn't work as long you aren't able to select/configure the key for your account.

I had this problem as well, and I figure out why---

The email account I was trying to add the key to was slightly different than what was in the key. For reasons I wont go into, we have two different forms of our email addresses: userID@example.com userID@email.example.com

Thunderbird was configured with the second form, but the key only had the first form. The key looked like it imported successfully, but when I revisited the end-to-end encryption screen, Thunderbird reported "Thunderbird doesn't have a personal OpenPGP key for userID@email.example.com"

I couldn't figure out how to add the additional email address in OpenPGP Key Manager, so I used Kleopatra.

Once the key had *both* addresses, Thunderbird imported the key successfully, the end-to-end encryption screen correctly reported they key was available for use, and I could use it to sign outgoing mail.

So, to summarize- my problem was the key I was importing didn't have a matching email address.