When you migrate a Lightroom Classic catalog to Lightroom (cloud), then Lightroom needs to copy the images first. That is a safety issue: Lightroom needs time to upload the originals, so after migration is 'done', it is not finished yet with the original images. To make sure that people do not delete them thinking that they don't need them locally anymore, they are copied.
Your internal hard drive does not have sufficient space however. That is what the error says. It would need an additional space of 32.77 GB. What you can do is go into the Lightroom preferences - Local Storage and change the location of 'Storage location of originals'. That is where Lightroom wants to store the copies temporarily, so set that location to an external drive that does have enough space.
" change the location of 'Storage location of originals'. "
I've already done it. I have an external hard drive with an empty space of about 18 terabytes. Did not help. I'm not sure I can free up so much data from my internal hard drive. This is too complicate for me.
P.S. "Additional Space Needed 32.77 GB , Available 48.01 GB" can be confusing, but what that means is that there is 48.01 GB available on your internal hard drive, but Lightroom needs 32.77 GB more. In other words, Lightroom needs 80.78 GB in total.