Apple Watch Series 6 storage only 26.6 GB?

All Series 6 watches have 32GB of internal storage. Same as Series 5 has. You do not have access to all 32GB though, because that storage includes the space taken up by the operating system, other system files, etc. So 26.6 GB is the total storage that's available to you. The 25 GB is what is left after what you have been using is installed. Put some more apps or music and that number will be reduced.

Don't forget 32 GB is the size of the storage before anything is installed. With the OS you lose space. The 26.6 GB is usable space not total space.