Which album are you browsing, when trying to delete a photo?
The Imports album does not always show the "Delete" option in the contextual menu in Photos 5.0 while Photos is still processing the recently imported photos.
And for the standard albums - there is a confusing change. Standard albums do no longer show the "Delete photo" command at all in the contextual menu, only "Remove from Album". If you are viewing a standard album hold down the command key ⌘to bring up the "Delete photo" command in the menues. The command key will change the menu entry "Remove from album" to "Delete ...". See: How to delete photos while viewing an Album in Photos 5.0 on Catalina - Apple Community
But you should still be able to use the key combination command-delete ⌘⌫ in any view to delete selected photos from the library.