iMac My keyboard is not being recognized

You don't mentioned if this keyboard is wireless or not, or if it is an Apple keyboard. In case both are true, see if the following article can help:

• If your Apple wireless mouse, keyboard, or trackpad isn't working with your Mac as expected - Apple Support

Yes, we are quite short of details. However, if this is a new computer with an Apple wireless keyboard, the most common reason for this is that no one expects a keyboard to have an on-off-switch: