I cleaned up my Mac to make space for the update which it said required 16GB. I had 21GB and after it completely downloads, it tells me it cannot install due to lack of space. Obviously after this download I am left with 10GB. What can I do?

you can purchase an external HDD or SSD of sufficient capacity and move your larger files (videos, pictures, documents, etc...) to it. once they are on the EHD, you can simply erase the same files from your internal drive. and, if it's just going to be used for storage, you can use an HDD and save a few bucks over the cost of an SSD. FWIW, i personally keep very little on my internal drive.

Optimize storage space on your Mac

How to free up storage space on your Mac

Melissa K. from the USA wrote:

The memory is super small (why do they make it so small?) Please help!

i'm thinking that you bought a base model. you could have configured it with more storage to meet your needs. apple doesn't force anyone to buy a mac with minimum storage.

I need to make space on my mac book for the latest update