Packard Bell 486DXz 66MHz

I am working on a Packard Bell 486 DXz 66MHz and I replaced the motherboard with a used that I think is good? Now when I turn the computer on it tells me that I have an; “invaild system disk, replace the disk and hit any key”. It won’t read the CD ROM. I am trying to boot with a WIN98 boot disk. HELP !!!!