I installed Tomcat and Tomcat Manager on a remote server as per the instructions on this post.
After adding it I successfully accessed the manager on http://I
I was using Netbeans to develop my code when I landed with the same error. I was using Tomcat 9.0.0.M19. I followed the suggestion provided by Dan and Jon above and changed my server path in Netbeans to point to the previous version of Tomcat i.e. 9.0.0.M18 and things started working.
You can download the previous version from http://archive.apache.org/dist/tomcat/tomcat-9/v9.0.0.M18/bin/
For MAC I downloaded http://archive.apache.org/dist/tomcat/tomcat-9/v9.0.0.M18/bin/apache-tomcat-9.0.0.M18.tar.gz
This should work.