iATKOS Inside: Just got a new computer and want to sell my old 2015 iMac running Sonoma. It’s telling me I cannot use the reset assistant due to the os being modified. Is there a way around this or do I have to redownload the old os to reset it?

Just got a new computer and want to sell my old 2015 iMac running Sonoma. It’s telling me I cannot use the reset assistant due to the os being modified. Is there a way around this or do I have to redownload the old os to reset it?