Still having a hard time understanding Icloud - why am I out of disk space with 1T available in the cloud?

I have read numerous posts. I have everything selected as optimized when it comes to storage space on my macbook pro. I have 2 TB (1TB available) for Icloud. I have an IMac with 2 TB as well. However my Macbook pro is 500GB. I tried to save a file on my mac to the icloud drive (or currently my desktop). The macbook says I do not have enough space and I need to delete files or optimize storage. However, I have over a TB available on icloud drive. This makes no sense. I could delete a large file from icloud and this would allow me to save the file to my computer (on Icloud). However, why would I want to delete a file from Icloud when I am paying for 2TB of storage. Is there a way to remove Icloud sync from my computer, yet, access it to upload files online?