In Gutenberg (WP 5) the button is only available in “Classic block”.
(My System: WP 5.2.4 German language)

On a fresh loaded page when first adding a new Classic block or first clicking within an existing Classic block I get a red error message “Failed to load plugin url: http://…/wp-content/plugins/clear-floats-button/mce/clear/langs/de.js” which has to be closed manually.
But afterwards the clear floats button is there and works as expected.

I’ve investigated the file path and found no “langs” folder in the “clear” folder.

Please repair your great plugin!
As the “langs” folder seams not necessary for the button to work, you could remove the reference to it in the plugin code.
Or add missing “langs” folder and “de.js” file.

And a second big wish: Please make the clear floats button available in regular Gutenberg blocks like paragraph and list!