Photos Rebuilding?

Please read the following user tip and restate your question/problem in detail: Writing an Effective Apple Support Communities Question - Apple Community  

Include:

What system and Photos versions are you running?

Are you running a "managed" or "referenced" library?

Where is your library located? If on an external HD how is that drive formatted and connected to your Mac?

How much free space do you have on your boot drive?

Have you run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

You must have a unique problem because this is the first time I've ever seen or heard about that particular

message.

Glad you got the Photos issue resolved. On another note there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus or security apps on your Mac.  This user tip describes what you need to know and do to protect your Mac: Effective defenses against malware and other threats - Apple Community. 

Most experienced contributors to these forums consider CMM akin to malware. With that in mind uninstall CleanMyMac according to the developer's instructions.  

You can check to see if you've removed all of the supporting files by downloading and running Find Any File to search for any files with the application's name  and the developer's name in the file name.  For CleanMyMac software you'd do the following searches: 

1 - Name contains cleanmymac

2 - Names contains macpaw

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

Lastly you should be aware the the pitfalls of a referenced library so you know what may be in store for you. Read this user tip: Disadvantages of a Referenced Library in Photos - Updated for macOS 10.15 Catalina. Just some food for thought.