Can't use photo stream on my iMac without upgrading iCloud Library?

I can't get my Photo stream to show up on my iMac since upgrading to Photos.

It seems that I have to buy storage for all my photos (18,000+) to get Photos to stream.

Works on my iPhone and iPad.

When I go to OS X preferences >iCloud Photos is unchecked.

When I try to check it, I get this message.

I have Library unchecked, so I should get streaming without signing up for P