I need to update my iMac (21.5-inch, Late 2009)from el Capitan to High Sierra

Im trying to update my iMac (21.5-inch, Late 2009)from El Capitan to High Sierra(or any newer version) but when I downloaded the newer ones from the app store it told me I needed 10.12 or something) So i backtracked to just after El Capitan and it will download but when I try to install it tells me "OS X El Capitan update can't be installed on this disk. This volume does not meet requirements for this update." Does this mean I need to free up space to download it? I updated it last year for the first time in at least 6 years to El Capitan so my con could use it from virtual school and it has worked great. We don't use it much now but I am needing to print out LOTS of paperwork for mother and when we hooked the new printer up it said we needed to update. Any and all help is appreciated, been at this for 15 hours and I'm getting frustrated thanks!