If the key you want to share does not appear under "doors" it's because you don't have the right to share the key to that door.
There can be two reasons for that:
The person who gave you the key shared it with you from the app. Keys shared from the app cannot be shared further.
The person who gave you access didn't give you the right to share keys.
To fix the issue you need to go back to the person who gave you access and ask them to add you in the control panel and give you the right to share keys from there. See here how it's done.