With WP-Juicebox (the dedicated Juicebox plugin for WordPress) installed, you should see something like the attached image when you create a new page or post (with the 'Add Juicebox Gallery' button in the toolbar at the top of the screen).
The screenshot was taken with WordPress 5.4.1, WP-Juicebox 184.108.40.206 and the WordPress Twenty Twenty theme active.
If you do not see the 'Add Juicebox Gallery' button, then please go to your plugins page and ensure that WP-Juicebox is installed and activated.
If the plugin is installed and activated but you still do not see the 'Add Juicebox Gallery' button, then there may be another plugin that you have installed and activated that is somehow interfering with WP-Juicebox's functionality.
If this is the case, then I would recommend that you try deactivating all plugins (other than WP-Juicebox) and then reactivate them one-by-one (checking WP-Juicebox's functionality after each one) to see if you can find the plugin causing the problem.
Regardless of whether of not you can see the 'Add Juicebox Gallery' button, you should still be able to create a gallery (on the 'WP-Juicebox -> Manage Galleries' page) and then manually enter the shortcode into a Gutenberg Shortcode Block.
Please confirm the following and we'll hopefully make some progress in troubleshooting your problem.
(1) Is WP-Juicebox listed as being installed and activated on your plugins page?
(2) Can you create a gallery successfully on the 'WP-Juicebox -> Manage Galleries' page?
(3) Are you able to add the shortcode (presented after Step #2 above) into a Gutenberg Shortcode Block?
If you are able to confirm/do all 3 steps above, then all that is remaining is to attach your gallery images to the page or post containing the gallery's shortcode.
I'll just copy and paste the instructions for doing so from the WP-Juicebox support page:
Add a Gutenberg 'Gallery' block to the post containing the WP-Juicebox gallery shortcode.
Upload images from your local machine by clicking the 'Media Library' button (in the 'Gallery' block) and dragging and dropping the images into the media window.
Wait until the images have finished uploading.
Reorder the images (if necessary) by going to the 'Create Gallery -> Media Library' section, selecting 'Uploaded to this post' from the drop-down menu and then dragging and dropping the images into the order you require.
Close the media window using the cross at the top right (do not click 'Create a new gallery').
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.)
I hope this helps.
Please let me know how you get on.