Can’t upgrade from macOS from Mojave to Big Sur: ”There's not enough free space on the selected volume.”

Question

I want to upgrade my macOS from Mojave to Big Sur. I've downloaded the OS update by following these steps:

1. Go to Apple.
2. Go to System Preferences.
3. Go to upgrade Software.
4. Click Upgrade Software.
5. Choose Big Sur.
6. Click Install Big Sur.
7. Agree the License.

However, I blocked to choose the available disk and I get this message:

“There's not enough free space on the selected volume to upgrade the os. An additional 12.87 GB is required.”

What should I do to free up the space and install Big Sur?

I've checked my available space on my system and this what I found:

1. Apps: 12.89 GB
2. System: 83.22 GB
3. Available: 22.61 GB

And this is the content of my drive:

I am confused why the system takes up 83.22 GB of space!