cannot install click packages

[Lukáš Tinkl]

Trying to install downloaded click package, I get the following failure:

unity8test@ltinkl-Inspiron-7548:~$ click install --allow-unauthenticated com.ubuntu.telegram_2.2.19.0_amd64.click
WARNING:root:Signature check failed, but installing anyway as requested
Cannot install com.ubuntu.telegram_2.2.19.0_amd64.click: Cannot acquire permission to write to /opt; either run as root with --user, or use "pkcon install-local" instead

Doing the same with "su":

unity8test@ltinkl-Inspiron-7548:~$ sudo click install com.ubuntu.telegram_2.2.19.0_amd64.click --allow-unauthenticated --user=unity8test
WARNING:root:Signature check failed, but installing anyway as requested
Cannot install com.ubuntu.telegram_2.2.19.0_amd64.click: Cannot acquire permission to write to /opt; either run as root with --user, or use "pkcon install-local" instead

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: click 0.4.43+16.04.20160203-0ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-24.43-generic 4.4.10
Uname: Linux 4.4.0-24-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
Date: Fri Jun 10 16:48:13 2016
InstallationDate: Installed on 2015-06-05 (371 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
SourcePackage: click
UpgradeStatus: Upgraded to xenial on 2015-12-08 (185 days ago)