Your software
My Mautic version is: 4.12
Your problem
My problem is: I tried to select an existing email in a campaign but its nowhere to be found. Please see video here which explains the problem: Recording #88
These errors are showing in the log: nothing
Steps I have tried to fix the problem: Recording #88
Hi,
You are selecting a Segment Email. (See the little chart? It’s a segment email)
Only Template Emails (Which are not the same as Email template 
just to make your life harder) can be selected in campaigns.
Duplicate the email you wanna use (lkittle dropdown next to the name) and choose “campaign email”.
Save.
Publish.
Now It’s available.
Mautic is hard. Take this free course it will get you through the first steps:
Thank you so much - your solution works well 