You get this message because the site cannot use your MP3 to make a video. The program that the site uses (FFMPEG) does not support sample rates of less than ~16000hz.
You will need to follow the instructions below to re-encode your mp3 so the sample rate is higher (44100hz is best):
If you do not have Audacity (an open-source music editor), you can download it from http://audacity.sourceforge.net/download/
Click File > Open - find the mp3 file you need to re-encode and click 'Open. The file should now be open in Audacity.
Look for the 'Project Rate (Hz)' setting in the bottom-left corner and set to 44100
Click File > Export
Select 'MP3' from the 'Save as Type' drop-down menu. If this is the first time you have used
Audacity to export an mp3, you will need to follow the LAME Installation instructions (below).
Click 'Options' and select a bitrate - 320kbps is the best quality (but also largest)
Go to the LAME download page - http://lame1.buanzo.com.ar
Under "For Audacity on Windows", left-click the link "Lame v3.99.3 for Windows.exe" and save the file anywhere on your computer. Do not right-click the link to the .exe file.
Double-click "Lame v3.99.3 for Windows.exe" to launch it (you can safely ignore any warnings that the "publisher could not be verified").
Follow the "Setup" instructions to install LAME for Audacity. Do not change the offered destination location of "C:\Program Files\Lame for Audacity".
The first time you use the "Export as MP3" command, Audacity will ask for the location of "lame_enc.dll". Navigate to "C:\Program Files\Lame for Audacity", select "lame_enc.dll", then click "Open" and "OK".