Thanks for this great module.
The code example given on the module page is faulty. The #type property should be changed to "rangeslider_range" as below
$form['example'] = [
'#type' => 'rangeslider_range',
'#title' => 'Rangeslider element',
'#min' => 5,
'#max' => 30,
'#step' => 2,
'#default_value' => 12,
'#description' => 'Example range slider form element.',
'#data-orientation' => 'horizontal',
'#output' => 'below',
];
Fixed
Documentation
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