Can someone tell me why I have only 11GB free??

The simple answer is you have a lot of files.

My question is where did you get a Mac with a 33.4GB disk? At that size, I'm, guessing it's a partition set up to hold data and you're simply running out of space. Or am I reading that wrong?

With such a tiny 128 GB Internal Drive, this is going the a challenge to keep the computer " lean " meaning stripped down to the minimum of essentials.

Yes, the tiny 128 GB drive should have never been available at time of purchase IMHO but it is what it use.

Now, you say the drive was erased by using this method from Apple " Use Disk Utility to erase an Intel-based Mac "

As already noted, it is a shame that macs with such small drives were sold (and for so long), but on to the matter at hand.

Select your drive in the Desktop, and press Command-I to open the Info window.

What do you see? It may say

Avaliable: 33GB (some amount purgeable)

The problem may be in that portion that is purgeable. It should be available when needed, but sometimes it is not.

Open Terminal and paste the following line:

tmutil listlocalsnapshots /

What do you get as the output of the above command?

Good points by @Luis Sequeira1