Well, that means you don't have it. Would it be possible for you to install it?
Well, you don't have that either. You'd have to install it. I think mutt's a full-out email client though, rather than something to use with sendmail.
There's probably lots and lots of them, but if you don't have uuencode, you're not likely to have anything else either. this might tell you how to make do with openssl's base64 encoder, by assembling your own mime headers.
Which Linux distro are you using? (e.g. Ubuntu, Fedora, PCLinuxOS, etc.)
You need to install whatever you want to use. If you are using Fedora, you can install mutt with this command.
Either use su to become root or preceed the command with sudo if that's set up:
yum install mutt
On Ubuntu, PCLinuxOS and some others use
apt-get install mutt
Other distros use other commands.
Using uuencode directly seems a bit laborious these days, but when I executed the command yum search uuencode, Fedora returned uulib-devel. So maybe it's in that package.