I can’t create a shared photo album.

sorry but we can not see you so you have to give us details

what instructions did you follow? What exactly happens?

From Photos Help

Create a shared album

It’s easy to create a shared album to share your photos and video clips.

Create a shared album

1. Select the photos and video clips you want to share.
2. Click the Share button in the toolbar and choose iCloud Photo Sharing, then click New Shared Album.
3. Type a name for the shared album, then type the email addresses of the people you want to share the album with.If you want subscribers to be able to view the shared album from an iOS device, be sure to use the email addresses associated with their iCloud accounts.
4. Click Create.

Your invitees are sent an email asking them to subscribe to your shared album.

LN

How is your Photos/iCloud preferences configured? Like this?

And you don't see it under Shared in the sidebar?

Delete that shared album and try creating a new one. Start by adding just one photo to it. Does it work?

Repair the library according to this Apple document: Repair Your Library and try again.

And when you look in the Photos albums section do you see the album?

LN