Anyway, I wonder how long I'll be able to use "WP Classic" and wouldn't I just
wind up right here again?
Yes, I agree. It would be better to get to grips with WordPress 5 and new Gutenberg Editor as this seems to be the way forward and WordPress are not going to drop the Gutenberg Editor any time soon.
Your screenshot shows that WP-Juicebox is installed and active and that you've managed to add a Juicebox gallery to your post.
All you need to do now is attach images to your post (using core WordPress functionality) and Juicebox will automatically display them in the gallery.
(1) Click the 'Add Block' button (the '+' in a circle near top-left in your screenshot) to add a new Gutenberg block to your page or post.
(2) Select a 'Gallery' block. (You'll find it in the 'Common Blocks' section.)
(3) Click the 'Media Library' button within the 'Gallery' block.
(4) Drag and drop the images that you want to attach to your page or post (i.e. the images that you would like to be displayed in your WP-Juicebox gallery) from your hard drive into the media window.
(5) Wait until the images have finished uploading.
(6) Reorder the images (if necessary) by going to the 'Create Gallery -> Media Library' section (within the media window), selecting 'Uploaded to this post' from the drop-down menu and then dragging and dropping the thumbnails into the order you require.
(7) Close the media window via the cross at the top right. (Do not click the 'Create a new gallery' as this will create a native WordPress gallery in addition to your WP-Juicebox gallery.)
(8) You can now safely remove the 'Gallery' block from the post. (Using a 'Gallery' block is just a convenient method of attaching images to a post.)
WordPress does not make it easy to attach images to a post (or to see images which are attached to a post) but the above method works.
If you find you still have trouble attaching images to a post, then there are a couple of alternatives.
(1) You could use a Flickr account as the source of images for your Juicebox gallery. WordPress would then have no part in the image management of your gallery.
(2) You could choose not to use WP-Juicebox at all and, instead, create a Juicebox gallery on your computer (with JuiceboxBuilder) and manually embed the gallery into your WordPress post using the baseUrl method documented here. Here is an example of how you could do this:
Create your gallery with JuiceboxBuilder and save it to a new empty folder named "my_gallery_folder".
Upload the entire gallery folder (not just the contents) to the root directory of your web server (using an FTP program such as Filezilla).
Create a new 'Custom HTML' Gutenberg block (in the 'Formatting' section) and paste the following embedding code (changing the gallery dimensions and background color if you like):
<!--START JUICEBOX EMBED-->
<!--END JUICEBOX EMBED-->
The leading slashes in the paths above denote your root directory so the code above will work without modification as long as your gallery folder is named "my_gallery_folder" and has been uploaded to your root directory.
Incidentally, you can upload your gallery folder to anywhere on your web server as long as the two paths within the baseUrl embedding code (the path to the 'juicebox.js' file and the baseUrl entry itself, pointing towards the gallery folder) are correct.
You can see there is nothing on the left to navigate to juicebox plug in.
You'll only see the 'WP-Juicebox -> Manage Galleries' link (shown in my screenshot in a post above) on your WordPress dashboard page (not on an 'edit a post' page such as the one shown in your most recent screenshot).
Just click the 'W' icon (the one in the circle at the top-left of your screenshot) to see the WordPress dashboard menu ath teh left of your screen. The WP-Juicebox link will be near the bottom of the list. (This will not help you to attach images to a post, though... it will only give you access to the 'Manage Galleries' page where you can edit galleries (to change configuration options) or delete galleries.)
You cannot even see there are photos associated with this editing page, yet when published, there actually IS a photo page.
That's down to the way WordPress works. The only way I've found to see the images attached to a post (whilst creating or editing a post) is to create a 'Gallery' block, check the images in the media window (accessed via the 'Gallery' block') and then remove the 'Gallery' block afterwards.
Otherwise, you can see which images are attached to which posts in the 'Media -> Library' itself (accessed via the WordPress dashboard).
I hope my notes above help somewhat.