I buy an m-audio m-track 2x2
Inside the box I found a Cubase EL 10 code
But After computer format, the old code will not be accepted
what should I do
That code was an activation code for your licens and as you have already used and activated that code it obviously doesn’t work anymore.
Your license should be tied to your MySteinberg account so it should be just a matter of installing the licenser software on your computer and login to your MySteinberg account and reactivate the licens.