Hello, Why am i not able to download the latest version of the software? My computer says I have OS X El Capitan version 10.11.6

If your Mac is a 2009 (i.e. "Core 2 Duo" or earlier) you won't be able to update the macOS past El Capitan due to age of the hardware. You can still use it for word processing and the like but only FireFox ESR will open most websites (browsers are no longer being updated). Additionally, El Capitan is too old to receive security updates from Apple. In the case of such outdated hardware/software, it will be necessary to be very careful online (don't click on attachments in emails, on web advertisements or attempt to download software outside the App Store). What I did in this situation — mainly because surfing the web is risky on outdated computers with unsupported browsers — was to buy a 2020 model (still some new Apple Silicone "M1" computers to be found on clearance). If cost is not a factor, buy an M2 (newest version). Be advised that if you do buy a new Mac, however, any software you had running on El Capitan (i.e. Microsoft) will not run on the M1/M2. In that case, you will also need to obtain updated software.

Oops. My bad. I found out since posting the above reply that the 2009 Core 2 Duo iMac can be upgraded to macOS Sierra or to High Sierra. All these years I assumed that because the App Store did not display an operating system more current than El Capitan it was because my hardware would not allow for it, but in this case I felt compelled to update this topic to clarify that it may be possible for a late 27" 2009 iMac to be updated after all (caveat being, even High Sierra is considered out of date too).

There is an Apple support document that explains how to create a bootable USB stick for the purpose of updating macOS, but it may be easier to go on the Big (A) Rainforest website and buy one pre-made (assuming you're okay with the risk associated with obtaining your macOS install source from a third party). More importantly, the aforementioned document contains links to older versions of macOS that the App Store will not let you view/download. (Apple doesn't make finding older macOS easy!)

Having now upgraded from El Capitan to High Sierra, I have run into serious upgrade-related problems with black screens and issues logging into iTunes and the App Store. If you're not prepared to troubleshoot installation issues — which are all too common, particularly on vintage hardware — buying a new(er) Mac with the current macOS (Ventura) is the better way to go.

Congratulations on the new baby.

You can find details about your particular machine model under the Apple Menu, About This Mac (do not share your Serial Number here) but if you can share the model information that will be helpful in determining which macOS versions are supported. You can also find that kind of detailed info yourself on www.everymac.com

For example: I have a Retina 4K, 21.5-inch, 2019 model iMac

What model do you have?

If the iMac in question is bootable, you probably don’t need to make a bootable USB installer flash drive.

How to download and install macOS - Apple Support

Re: “it may be possible for a late 27" 2009 iMac to be updated after all”

My 27” Late 2009 iMac runs High Sierra.

What model and year is your Mac?