my imac wont let me install a later mac os version from its current version 10.11.6 is there a reason why?

Does your iMac meet the system requirements for a later version of macOS?

iMac (early 2009) can only use up to macOS 10.11

iMac (late 2009) to iMac (late 2011) can use up to macOS 10.13

iMac (late 2012) to iMac (late 2013) can use up to macOS 10.15

iMac (2014)+ can use macOS 11.0.1 Big Sur (latest version of macOS)

You can find links & instructions for downloading these macOS installers and for creating a bootable macOS USB installer (recommended):

https://support.apple.com/en-us/HT201372