You've got a typo in the repository's URL. The domain needs to be updates.signal.org
, and then the path starts with /desktop
, so you have
https://updates.signal.org/desktop/apt
You are trying to reach the domain updates.signal.org.desktop
, which doesn't exist, and then look for a path /apt
. Please note the difference in the following (wrong) URL to the correct one above
https://updates.signal.org.desktop/apt
If you use the correct URL, the repository should work as expected.
In general, you want to be really precise in cases like this. In your question, you had an additional typo, writing https//
instead of https://
. But this part seems to be correct, according to the linked screenshot. It's a bit like with phone numbers, you need to get every one of the digits right, and all in the right order too ;)