Old iPhoto library won't support on latest Photos App

So what happens when you follow the recommendation in the message ("use Library Import")? That is, import from the old library into your new library, not try to open the old library.

See "Import from another photo library on Mac" in the Photos User Guide available through the Help menu in Photos:

If you use multiple libraries, you can import photos from one library into another.

Open Photos for me

1. Go to the Photos app on your Mac.
2. Make sure the library you want to import photos into is currently open.
3. If it’s not, you need to switch libraries. Choose Photos > Quit Photos. Press and hold the Option key, then click the Photos icon in the Dock. In the Choose Library dialog, select the library you want to import photos into, then click Choose Library.
4. Choose File > Import, navigate to the location of the library you want to import from, select it, then click Review for Import.
5. Note: If you participate in an iCloud Shared Photo Library, click the Library pop-up menu in the toolbar and choose the library to import into.
6. Do one of the following:

• • Import the entire library: Click Import All New Items.
  • Import selected photos: Select the photos to import, then click Import Selected.

You have two options to getting your iPhoto library into a new or existing Photos library:

1 - use the Photos import option to import the iPhoto library into the Photos library.

2 - use the paid version of PowerPhotos to merge the iPhoto library into a selected Photos library.

Option is a better choice especially if you have a lot of organizational effort put into the iPhoto library. I ran a number of tests with a 3119 photo, 72 video and 97 keyword Photos library with keywords, captions, keywords and locations.  I merged it into an empty library using Photos and PowerPhotos.  This is what I found was imported by each;

PowerPhotos:           Photos:

Albums***                                          -----------

---------------  Folders

Smart Albums (as regular albums)** -----------

Captions                                             Captions

Titles                                                   Titles

Keywords* Keywords (some but not all: …

…see Note *)

Original images                                  Original images

Edited images                                     Edited images

---------------                                        Locations

Favorites                                             Favorites

Can detect and exclude                      Duplicates

duplicates upon import or not

Neither method could import/merge projects. 

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library. 

Notes:

*The Photos app only imported 84 keywords our of 97. PowerPhotos merged 147 keywords. Photos only imported keywords that were embedded in the original file and not those added by Photos. PowerPhotos imported all of them.

**The original library had 90 Smart Albums. All were merged by PowerPhotos but, as indicated above, the Smart albums were brought over as regular albums. No album of any kind were imported by Photos. Also only PowerPhotos imported folders.

The original library had 34 regular albums and 1 folder with some nested albums. PowerPhotos merger all of them successfully.

The original library had 72 videos. Photos imported only 57. PowerPhoto got all 72 in its merge. The above is from user tip Comparison of PowerPhotos vs Photos re Im… - Apple Community

Perhaps you would have better luck with PowerPhotos, as Old Toad suggested.

It is also possible that your old iPhoto library has some corruption that makes it unusable.

You might want to use markwnsn's excellent advice on a new, empty library, so that you don't put these images into your current system library. Also, they won't go into iCloud if you use it. You can easily import them to another library later.

Create additional photo libraries in Photos on Mac - Apple Support