Why I obtain this error message when I try to add the TomCat installation into NetBeans on Ubuntu (The CATALINA_HOME /conf/server.xml can't be read)?

I have installed TomCat 8 on an Ubuntu 15.04 system and I have to use NetBeans

Now the problem is when I try to add my TomCat 8 installation into NetBeans (into Window --> Services --> Servers --> Add Server). After that, when I select my TomCat directory it gives me the following error message:

The CATALINA_HOME /conf/server.xml can't be read.

Why? What does it mean? How can I fix this issue?