i tried every type from export but can’t play it on my computer. i tried to import back the “exported file” to AUDACITY but even that, it said : unknow file… Help me 
Try exporting again but this time use a very plain, short filename as a test. No punctuation and no spaces. Use WAV, not MP3. Did that work?
Where are you trying to export to? Try forcing the export to your desktop instead of wherever Audacity wants to put it.
Koz