Cannot enable iCloud photo library

I recently upgraded my iCloud plan for more space in order to give iCloud photo library a try.

My library is 100 Go, I chose to keep originals on my mac and after uploading 30 Go of data it stuck for several hours.

As seen in other posts, a potential solution was to disable and enable iCloud photo library.

The problem is that when I try to re-enable it complains that there is not enough space on my mac (I have 50 Go of free space) and imposes the "Optimized storage" option.

Why does it complain about space ? There is enough available space on my mac and all the photos in iCloud are already in Photos library.

Is there any way to fix this as I want to keep originals on my mac ?

Thanks