IMAC 2011 can i down load catalina 10.15.1

No, Catalina is not compatible with your hardware: macOS Catalina is compatible with these computers - Apple Support

iMac

Learn how to identify your iMac.

• 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)
• iMac (27-inch, Late 2013)
• iMac (21.5-inch, Late 2013)
• iMac (27-inch, Late 2012)
• iMac (21.5-inch, Late 2012)

High Sierra is as far as your Mac can go.