Tomcat and Eclipse Integration Error: "Unknown version of Tomcat was specified."

I suggest installing standalone Tomcat into /usr/local instead of using the one in /usr/share directory.

cd ~/Downloads
wget http://apache-mirror.rbc.ru/pub/apache/tomcat/tomcat-7/v7.0.52/bin/apache-tomcat-7.0.52.tar.gz
tar -xzf apache-tomcat-7.0.52.tar.gz
sudo mv apache-tomcat7.0.52 /usr/local/apache-tomcat7.0.52

Then use /usr/local/apache-tomcat7.0.52.

Make sure that the Tomcat folder you're pointing to contains the following files:

conf/catalina.policy
conf/server.xml
conf/web.xml
conf/context.xml
conf/tomcat-users.xml
conf/catalina.policy
conf/catalina.properties
lib/catalina.jar

as Eclipse is scanning for them.

Here is a Linux command which can help you to find that folder:

find /opt /usr '(' -name catalina.policy -o -name tomcat-users.xml -o -name catalina.properties ')' -exec sh -c 'dirname $(dirname {})' ';' | uniq

In Ubuntu the application doesn't have default permission to read/edit all the folders.

I realized that the permissions for the TOMCAT installation directory were not set correctly.

I changed it to 755 and it worked.

sudo chmod -R 755 /opt/tomcat/apache-tomcat-9.0.24/