I have RedHat Linux and I want to create a spare boot disk. The documentation says to use the mkbootdisk command, but my computer does not have that command...but it does have the man page for the command. What is wrong?

Have you tried typing the full command or location of where the make boot disk command resides. Search it and then try to type the full path and see if that works.

Thanks for the help. I found out that I had the man page, but there was no command.

mkbootdisk probably needs root privileges.
Try to log in as root.
If you are using the su command then you will have to type the complete path. Try 'whereis mkbootdisk'