Code: Select all

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Starting pkgProblemResolver with broken count: 1
Starting 2 pkgProblemResolver with broken count: 1
Investigating (0) telegram-desktop:amd64 < none -> 1.2.17-1 @un puN Ib >
Broken telegram-desktop:amd64 Depends on libtgvoip1.0.3:amd64 < none | 1.0.3-3 @un uH > (>= 1.0)
  Considering libtgvoip1.0.3:amd64 0 as a solution to telegram-desktop:amd64 9999
  Re-Instated libtgvoip1.0.3:amd64
Broken telegram-desktop:amd64 Depends on qtbase-abi-5-9-5:amd64 < none @un H >
  Considering libqt5core5a:amd64 3246 as a solution to telegram-desktop:amd64 9999
  Considering libqt5core5a:amd64 3246 as a solution to telegram-desktop:amd64 9999
Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 telegram-desktop : Depends: qtbase-abi-5-9-5
E: Unable to correct problems, you have held broken packages.


Can someone guide me what I am doing wrong

You are not doing anything wrong.
Neon uses newer Qt libraries than Ubuntu 18.04, so a small number of packages that use Qt won't install as their packaging requires the specific Qt version they have in 18.04.

Go grab a much more up top date version of Telegram from their website. It will not have this issue at all.
Or use the flatpak or snap version.