If you have Stackable installed and activated and you can't see the blocks, there's a chance that you are editing your page using the Classic Editor.
Stackable blocks can only be used in the Block Editor / Gutenberg, you will have to edit your pages using the Block Editor in order to see our blocks.
If you're using the Class Editor plugin activated, you can enable switching between the Classic Editor and the Block Editor by going into Settings > Writing, then make sure you have the option Allow users to switch editors in Yes
When you have this option to Yes, you can click the Switch to the Block Editor link when editing in the Classic Editor.
To go back to the Classic Editor, you can click on the More Tools & Options button on the upper right corner of the screen (the one that has 3 dots) then click on the Switch to Classic Editor option.