If you cannot find your email, contact Steinberg Support with your proof of purchase to have the code reissued.
This usually means the code has already been converted into a license. Check your "MySteinberg" account online to see if the license is already registered. activation code for cubase 11
If you bought a physical copy, look for a "Download Information" sheet inside the box. Your Download Access Code (DAC) is printed here, which you must exchange for an activation code. If you cannot find your email, contact Steinberg