An error occurred while saving one or more files. An error occurred while opening a codec.
First Read the 'Sticky' at the head for the main Sound Forge forum page and state the pertinent info needed to help you..
Do you have the latest update build installed for the version you have?
Resetting Sound Forge has been known to fix the MP3 issue so I would try that.
In the 'File' menu, select "Reset and clear cached data". Close and restart Sound Forge
Alternate method: Hold the "Ctrl+Shift" keys whilst launching Sound Forge will exhibit the same reset dialog.
btw, welcome to the Magix Sound Forge users community @Jimmie-Gray.