Welcome to the Magix Sound Forge users forum @Charles-Jackson1229.
FYI, this is a user-to-user forum and no one here is a Magix employee including the moderators.

The lack of the MP3 codec is unfortunately relatively common.
Your PC must be on on-line for licensing when SF is installed, activated and the first time you selected or attempt to encode an MP3.
Assuming you have the 64 bit version installed, look in your "Program Files> SOUND FORGE> Sound Forge (#)> FileIO Plug-Ins" folder to see if there is a "mp3plug2" folder with a <mp3plug2.dll> file in it. If that is missing it can be downloaded. Otherwise a reinstall is usually a remedy. If that did not work, I recommend uninstall / reinstall.. but uninstall using a third-party uninstaller that can remove left-over registry entries and other pertinent data the OS uninstall leaves behind. You also want to temporarily disable anti-virus software when you reinstall
Revo Uninstall and Geek Uninstall are two uninstaller/cleaners that have pro and freeware versions you can download.
If that does not resolve it, request a Magix tech support ticket. It is confusing and with many steps.
- Select the "SUPPORT" at the top of the forum page
- Choose your product (Sound Forge Pro for instance)
- Click the "Ask Support" box on bottom right of page
- Under "I NEED HELP WITH" >> choose "Music" or other generalized category and ignore the "Perhaps this will help you" option
- Further down on the page choose your product (again):
- At the bottom-right, click the blue button "SEARCH FOR OTHER SOLUTIONS"
- Enter a keyword, such as 'MP3' and click the "FIND A SOLUTION" button
- Scroll down to the bottom of the page and select "SEND A SUPPORT REQUEST"
- Log into your account (again) if necessary
- If so desired, you can change the subject text (which is the previously entered keyword)
- Describe your issue as detailed as possible in the text box
- Click Submit and OK
- You will receive a confirmation email with a support ticket # .