Eureka! It is fixed.
I researched what is lxml and I found out that it is a phython lybrary. I checked in my system and I realized that I did not have it installed. After installing the package phyton3-lxml V 4.2.1-1ubuntu0.1 everything is working correctly. I researched and it seams that it is automatically installed by Inkscape at least in Windows that is why i don’t understand why it was not installed automatically in Linux Mint or why I did not have it installed before in my system. I hope this will help other people having the same problem, as I was.
By the way the extension Gear is also working now.
Thanks a lot Windell!!