No Cure I am afraid.  So I made a comprehensive summary of EXACTLY what I did:

Downloaded:    JuiceboxBuilder-Pro
Installed as:    "C:\Program Files (x86)\JuiceboxBuilder-Pro\JuiceboxBuilder-Pro.exe"

Created a Desktop Shortcut which opened the intial "To get started, . . ." page

Clicked on:    "New Gallery" button
Presented with a new page that included "DROP IMAGES HERE"

Clicked:    "Browse" . > Opened File Explorer.
Selected:    4 png files, each 720 x 1080 pixels
Hit Enter:
Screen Showed the four images as square thumbnails
Clicked on the first  which brought up a Pop-up (Title, Caption, Link)
Added Titles:    "Hello Everyone"; "1","2" & "3"

Clicked:    "Customize"
Clicked on    "Gallery Title" and inserted "Trial 1000".
Hit "Enter" and a working Gallery showed up in the working area
       
Left all other Customizations alone

Clicked on:    "Publish" which brought up "Save Gallery"

Gallery Folder Box showed;     C.\Users\[my user name]\Desktop\JUICEBOX GALLERY
Index Page Name showed:     index.html            

Clicked on "SAVE" button

Got a pop-up "Overwrite Folder?" message. Clicked "No"
Went back and changed Index.html to index1000.html (No folder with name 1000 had been entered or existed)
Got the "Overwrite Folder" message again.
Clicked on "Yes"
Nothing seemed to happen (But File Explorer showed that index1000.html had been saved)

Quit Programme

Re-opened Programme from Desktop shortcut

Clicked on:    "Open Gallery"
New Screen:    Address: Desktop > JUICEBOX GALLERY
        Showed three folder icons - images, jbcore, thumbs.
        "Select Folder"
        "Cancel"           

Clicked on:    "Select Folder"
   
Programme returned to "To get started, . . . " page.

Nothing would open the saved gallery.

Opened Windows 11 File Explorer.
Navigated to: C.\Users\[my user name]\Desktop\JUICEBOX GALLERY
Screen showed:   
        Folder Icons:      images, jbcore, thumbs
        Files:        config.xml, index.html, desktop.ini
                Trial 1000 (but with no file type suffix)

Clicked on Trial 100 - nothing happened
Double-clicked on Trial 1000 - Invitied to select programme (app) with which to open it.
Selected: JuiceBox-Pro.exe
Returned me to the start page "To get started"
Clicked on "Open Gallery"
Page opened showing just the three icon folders - images, jbcore and thumbs plus "Slect Gallery" which just returned me to the opening page "To get started"
WHERE DID I GO WRONG?