why no update for 10.11 os

If there is no software update available, it could be that this Mac is not eligible for updates anymore. Please go to apple logo In menu bar>about this Mac to see your Macs name and model year to see if you can update any further

For 10.12 you will need to have one of these Macs

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

For 10.13 you will need to have one of these Macs

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

For 10.14 you will need to have one of these Macs

• MacBook (Early 2015 or newer)
• MacBook Air (Mid 2012 or newer)
• MacBook Pro (Mid 2012 or newer)
• Mac mini (Late 2012 or newer)
• iMac (Late 2012 or newer)
• iMac Pro (2017)
• Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

And see this Link for the requirements of 10.15

macOS Catalina is compatible with these computers - Apple Support

The macOS version you can upgrade to depends on the year your Mac was released.

Click the Apple menu () at the top of your screen and choose About This Mac.