Hi,
first of all thanks for the response.
In order for the client to enable orders in his store I installed Variation Swatches for WooCommerce (https://el.www.ads-software.com/plugins/woo-variation-swatches/). With the plugin the drop down list is converted to button (thats why when you tested the test product you dindt find any issue).
Without the plugin, the drop down list for the first variation is desappearing.
I tried deactivated all the plugin from the site to check if any of the activated plugins is causing the conflict but the problem remains.
WooCommerce > Status–>https://pastebin.com/hGAVqVZB
You can see a small screen capture here (https://1drv.ms/u/s!AqREQcOFfgG0iJNDfICN-ZJ6j2f-fA?e=JrFXqk).