volume for disk not APFS. what does this mean?? how do i fix this?

Be thankful that is all it said.

iMac (20-inch, Mid 2009) goes not Qualify for Big Sur

iMac - Source >>>> https://support.apple.com/en-ca/HT211238

Learn how to identify your iMac.

• iMac (Retina 5K, 27-inch, 2020)
• iMac (Retina 5K, 27-inch, 2019)
• iMac (Retina 4K, 21.5-inch, 2019)
• iMac (Retina 5K, 27-inch, 2017)
• iMac (Retina 4K, 21.5-inch, 2017)
• iMac (21.5-inch, 2017)
• iMac (Retina 5K, 27-inch, Late 2015)
• iMac (Retina 4K, 21.5-inch, Late 2015)
• iMac (21.5-inch, Late 2015)
• iMac (Retina 5K, 27-inch, Mid 2015)
• iMac (Retina 5K, 27-inch, Late 2014)
• iMac (21.5-inch, Mid 2014)

Welcome and Thank you too :-))

What version of macX is it running now - as there might be some antique macX upgrade available