[Option] Alternative shorter shortcodes doesn't work

That means the shortcode is already registered by some other plugin. Common ones that offer [tabs] shortcode are synved shortcodes, shortcodes ultimate etc.

WP UI does not forcibly remove registered shortcodes by default, So if you have such plugins installed, it’s recommended to use wptabs shortcode. On the contrary you can use [tabname] and [tabcont] within wptabs.