In my case, the problem was in the web.xml file, where i forgot to precede the url-pattern with a "/" (e.g. /login.do).
What solved my problem was, as mentioned above by @Ajak6, by adding the servlet API as an external JAR library for the project.
However, I found a better way, by adding Tomcat to the target runtime as follow;
1) project > properties
2) on the side menu select "Targeted runtimes"
3) select Tomcat (click apply)
4) click ok