Monterey macOS 12.1

Seem like your SSD is only 128GB. Thus you can't actually stor much internally. You will have to use iCloud or store soe stuff on an external disk.

• If upgrading from macOS Sierra or later, your Mac needs 26GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44GB of available storage.

See:

Free up storage space on your Mac - Apple Support

Kayujay wrote:

I'm trying to download macOS Monterey and it's saying I don't have enough space on the computer.

I'm only using 4 or 128GB. What's could be the problem?

?

your system files alone take up more than 4...so I have no idea here. If you hightlight your Macintosh HD in the FInder side bar Command i will open the info pane:

You can try from a a SafeBoot https://support.apple.com/en-us/HT201262

How to upgrade to macOS Monterey

https://support.apple.com/en-us/HT212735

verify you are using the Safari browser to download/initiate the macOS links.

Kayujay wrote:

I'm trying to download macOS Monterey and it's saying I don't have enough space on the computer. I'm only using 4 or 128GB. What's could be the problem?

Am guessing that this computer has 4 GB RAM and the tiny 128 GB Internal Drive/ If that is correct - it maybe possible to install Monterey but will very very difficult to upGrade and then maintain at least15% the the Total Drive Capacity as Free and Empty Space. That is often time consider as Good Computer practice. Otherwise, the drive gets too full and may even reach a critical state with the computer will cease to function.

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

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.