If you do not have the system software. you can find it on eBay by searching for
+
Apple Mac OS X 10.4 Tiger - Full Version for Mac
+
Here's one https://www.ebay.com/itm/Apple-Mac-OS-X-10-4-Tiger-Full-Version-for-Mac/332665816922?epid=127620286&hash=item4d746d875a:g:WLwAAOSwLnBX-Sx2:sc:USPSFirstClass!79423!US!-1
If you do not have the system software. you can find it on eBay by searching for
+
Apple Mac OS X 10.4 Tiger - Full Version for Mac
+
Here's one https://www.ebay.com/itm/Apple-Mac-OS-X-10-4-Tiger-Full-Version-for-Mac/332665816922?epid=127620286&hash=item4d746d875a:g:WLwAAOSwLnBX-Sx2:sc:USPSFirstClass!79423!US!-1
Your hard drive cannot find an appropriate OS. Let's start here: go to this kink and identify exactly which machine you have:
https://everymac.com/ultimate-mac-lookup/?search_keywords=m5521