The library could not be opened (4302)

This will not work. You have two options depending on whether you'd like to do it manually or get a single archive with all your photos:

Option 1: Request a copy of all your iCloud Photos

You can do this as part of the process to Get a copy of the data associated with your Apple ID account.

1. Sign in to privacy.apple.com on a Mac, PC, iPhone, or iPad.
2. Select "Request a copy of your data."
3. On the following page, select the data you’d like to download (scroll down for iCloud Photos), then click Continue. You may not see iCloud Photos on the list of you have Advanced Data Protection on enabled.
4. Apple will notify you when your data is ready. This process can take up to seven days. You can check the status of your request at any time by visiting privacy.apple.com/account.

Option 2: Export photos manually from the Photos app on Mac

You can Export photos, videos, and slideshows from Photos on Mac in two formats:

To export photos in a specific format:

1. In the Photos app on your Mac, select the item you want to export.
2. Choose File > Export > Export [number] Photos.
3. Click the Photo Kind pop-up menu and choose the file type for the exported photos.
   1. JPEG creates small-size files suitable for use with websites and other photo apps. 
   2. TIFF is a lossless file format that works well for transferring photos between computers and apps.
   3. PNG is another lossless file format popular for web images.
4. Click the File Name pop-up menu and choose how to name the exported files (this step is optional).
   1. You can name your exported files using the titles you’ve given them in Photos, their filenames, or a filename numbered sequentially. If you choose Sequential, you can specify text to appear before each number (for example, Birthday - 1, Birthday - 2, and so on). 
5. Click the Subfolder Format pop-up menu and choose how to divide the exported files into folders.
   1. For example, if you export photos from several different days, you can have Photos export each day as a subfolder named after the day. Choose None to export the photos as individual files without subfolders.
6. Click Export.
7. Choose where to save the exported photo files, then click Export.

To export photos in their original format:

1. In the Photos app on your Mac, select the photos you want to export.
2. Choose File > Export > Export Unmodified Original.
3. If you want to export IPTC information (assigned IPTC metadata and keywords) as a sidecar XMP file, select the Export IPTC as XMP checkbox. 
   1. 
4. Click the File Name pop-up menu and choose how to name the exported files (this step is optional).
   1. You can name your exported files using the titles you’ve given them in Photos, their filenames, or a filename numbered sequentially. If you choose Sequential, you can specify text to appear before each number (for example, Birthday - 1, Birthday - 2, and so on). 
5. Click the Subfolder Format pop-up menu and choose how to divide the exported files into folders.
   1. For example, if you export photos from several different days, you can have Photos export each day as a subfolder named after the day. Choose None to export the photos as individual files without subfolders.
6. Click Export.
7. Choose where to save the exported photo files, then click Export Originals.

Do you have an external drive that you could use? You need to be able to set the "Ignore Ownership this volume" flag to be able to access a photos library created by a different user account.

• Prepare the external drive described as here: Move your Photos library to save space on your Mac - Apple Support

• Then copy the Photos Library to this drive (don't move it).
• Now log into the other user account and double click the copied library to open it. If that succeeds, quit photos, then copy the library from the external drive to the Pictures folder in the other user account.

This should work, if the Photos Library in question is not an iCloud Photos Library with the option "Optimize Mac Storage" enabled. You can only move a Photos Library this way, if all originals have been downloaded to the library, and if both user accounts ar on Macs with the same system version.