Error when opening Photos.app: "The Library Could Not Be Opened"

Did you boot normally and try the library again?

If the library now opens OK in normal mode cleaning the caches probably did the trick. If it downbeat then thee's one 3rd party software that's interfering with Photos. In that case download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

Badlydrawnboy wrote:

I tried to open Photos just now on my Macbook Pro and got this error:

I am able to open Photos on my iPhone and iPad.

What now?

Those are two entirely different and separate libraries and apps. They operated for the most part indecently from each other and the Photos on the Mac.

Try the following in order as needed:

1 - As a test launch Photos with the Option key held down and try to create a new, test library.  If successful close that library and try your original library again.

2 - after trying your original library again close Photos and reopen with the Option+Command keys held down an start a repair of your library.

3 - if the 2 fixes above fail boot into Safe Mode according to How to use safe mode on your Mac and and try the original library from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Note: am I correct to assume that your library is located on your boot drive in the Pictures offer?

What has changed? Did you just upgrade to a new system version?

Then you may want to check, if your Photos Library is stored on a suitable drive. After a system upgrade Photos may no longer tolerate, if the Photos Library is stored on an incompatible drive. If you are keeping the library on an external volume, check the file system format. It should be MacOS Extended (Journaled) or APFS. The external drive should have a wired connection, and not be a network share. It should not have been used for Time Machine or have other backup software installed.

See: Move your Photos library to save space on your Mac - Apple Support