Since viber uses libcurl3 instead of libcurl4 , when installing it on Ubuntu 18.4 some problems arise in keeping Virtualbox and Opera in the same pc (Ubuntu 18.04).
The following method can be used to eliminate the above problem
Assuming the viber.rpm file is being downloaded to Downloads folder of the PC.
Open the Ubuntu terminal and run following commands
sudo apt install alien
Making a deb file
sudo alien –to-deb –scripts viber.rpm
After the deb file is formed you can see the file in the Downloads folder as in the following figure
Then install the package
sudo dpkg -i viber_126.96.36.1995-3_amd64.deb
Now you can see the viber is being installed properly on the pc without harming other software like VirtualBox installed on the same pc