can not upgrade my iMac with montery

Try repairing your disk first and then upgrading https://support.apple.com/en-ca/guide/disk-utility/dskutl1040/18.0/mac/10.14

hope this helps

what error is it showing ?

you are lucky, I wish I was not able to install it, it basically killed my laptop,

I spent the last two days looking for a fix...