The error message is fairly clear 'Plugin installed in wrong folder'.
Look in your version.php file (and for installation errors it is usually helpful to reproduce the contents of your version.php in the forum post).
Find the line: $plugin->component = 'block_something_or_other';
If the component name is 'block_something_or_other', then it will need to be installed in 'blocks/something_or_other'. If the component is 'block_simple_html', then it will need installing in 'blocks/simple_html'.
Note the 'component' name should also match the name of the main file for the plugin (e.g. blocks/simple_html/block_simple_html.php) and the language file (e.g. blocks/simple_html/lang/en/block_simple_html.php)