Welcome to LQ!
As a user you need to have permissions to perform certain actions. The command is not in your 'PATH'. '/sbin/mke2fs' is where the command resides. A normal user should never need to issue or use this command. Potential problems with a normal user issuing the command with possible system conflicts or formatting of filesystem(S).
As stated you could use 'su' which would require the root password or use 'sudo' if the root setup the command to allow you the operation.
Just a few links to aid you to gaining some understanding;
Linux Documentation Project
Rute Tutorial & Exposition
Linux Command Guide
Utimate Linux Newbie Guide
Getting Started with Linux
Advanced Bash-Scripting Guide
Linux Home Networking
The above links and others can be found at 'Slackware-Links
'. More than just SlackwareŽ links!