resetting nvram pram on mac
Try starting while holding the Command-Option-Shift-Delete keys all at the same time and see if it will let you startup/install.
Do you have any external drives you could install Tiger on first?
If the above doesn’t work, try this:
1. Start Mac and hold Option-Command-O-F keys
2. When the grey screen gets loaded type reset-nvram then press Return key
3. At next prompt type reset-all and the Mac will restart
See if that will allow install.