Why can't I see a Get button for Big Sur?

Because Apple only keeps the most current version of the operating system in the Mac App Store (Monterey 12.2.1). The age of your Mac may limit your ability to upgrade to Big Sur as you would need a mid-2013 or later Air with a minimum of 8GB RAM and ideally larger than 128 GB internal drive. You would regret doing this on an Air with only 4GB RAM.

Understand that Mojave is the last operating system to support 32-bit applications or 32-bit printer/scanner drivers. Use the free Go64 application to get a list of your 32-bit dependencies, and from there, check with the third-party application vendors for 64-bit applications supported on Big Sur. Same drill for your printer/scanner vendor offering Big Sur specific drivers released in 2021 or 2022.

Once you have that sorted, remove any anti-virus or so-called Mac cleaner applications, then perform a final Time Machine backup of your Mac. Do not reuse that drive in Big Sur.

Now, using only Safari, visit How to Get Old Versions of macOS and provided your Air is hardware compatible, follow the link for Big Sur to its location in the Mac App Store. When it downloads, the installer will land in your /Applications folder.

During the upgrade process, keep your Air on a power supply and stable network connection.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

macOS Big Sur 11 hardware requirements

macOS Catalina 10.15 hardware requirements

Caution - jumping over all the newer versions and directly to Big Sur  / Monterey on Qualifying computers is Technically possible - there are those who have attempted and were successful. Then there are those who attempted and had less than good results.

If this is a Production or Mission Critical Set-up - a more prudent and measure approach is suggested. 

The above is from observations on the Apple Support Community Forums and IMHO

Tips Big Sir Upgrade Released Nov 2020

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Be aware - since you have marked your own reply as " Solved " this will generally indicate to all others looking to help - there is no need to help as it is Solved.

Regardless, there is no was to Add Storage to the existing Internal Drive except of one changes the drive to something with a Larger Capacity. So computer as of a certain Ade can have this done while others have the Internal Drive soldered to the LogicBoard making that impossible

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

The final word from Apple on Managing the " Other " Category

Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

there is no Get or Download button or link

Are you sure your Mac is compatible?

I have seen another post that said there wasn’t a Get button, but when I go to that link, there is a Get button. If your Mac is compatible, there may be a problem with the App Store.

I've read quite a few posts like yours where those links won't work. It seems that for some people they can only get to the Mac App Store Preview page and no further. According to Niel, a well-known poster here (if I understand him correctly) if you remove just the "https" at the beginning of the address in the address bar, and type in "macappstore" (without the quotation marks) in place of "https," then refresh the page or hit return, that will get you to where you can get the download. Please post back as to whether or not this works for you.

Thoughts on adding storage?

Just to make it clear: you cannot add storage. You can remove/delete/move some files by getting an external hard drive as "auxiliary" storage, but your internal storage space cannot be increased - it will always be the same as when you bought it.

Click here and follow the instructions.

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Change the https in the page's address to macappstore.

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