Cant update, and storage full of system files

RArna wrote:

Hei,

I got a MacBook Air (11-inch, Early 2014), 121,12GB. It is currently on macOS High Sierra Versjon 10.13.6, and have not been updated for a while. Is this the latest update for this MacBook, or am I missing something?

You are. It can be upgraded up to macOS Big Sur.

Ref: macOS Big Sur is compatible with these computers - Apple Support

Is it possible to remove some of the old system files that dont need to be there, to free up some space?

Potentially, yes.

Ref:

• Free up storage space - Apple Support
• What is “Other” storage on a Mac, and how… - Apple Community

The second link I provided definitely does cover "system" files. Please re-read it again.

Regardless, system files are managed by macOS. System includes:

• Time Machine local snapshots, but are marked as 'purgeable' and should not be removed manually. However, if you wish to do so, use the following commands in the Terminal app:
  • To get a list of current snapshots: sudo tmutil listlocalsnapshots /
  • To delete individual snapshots: tmutil deletelocalsnapshots <enter the snapshot>
• Spotlight Indexing
  • Rebuild the Spotlight index on your Mac - Apple Support
• iCloud Storage of infrequently-used files.

Try booting up your Mac in Safe Mode (to reduce the amount of "System Data" storage used,) and then, try upgrading it again.

You need to clear some storage so you can install any wanted updates. Also, if the HD gets too full the Mac will stop functioning and then you have a serious problem. Purchase an external disk so you can copy files onto it and then delete them from the HD to clear needed storage space. These disks are reasonably priced and easily connected with a USB cable normally provided by the vendor. Sometimes they need to be reformatted before use. If so, only use Disk Utility to do so, not any maintenance app that comes with the disk. They always cause problems for Macs.