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Help: Can't layer an element at the top of another one (mobile version issue)

Hi everyone,

I am trying to make my nav menu element to layer at the top of my vertical filtering but it doesn’t work. I already checked the courses on Webflow University but I am stuck. I only facing the issue on the category template page on mobile version. Could you help me out, please? Many thanks

Screenshot attached
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Here is my site Read-Only:

https://preview.webflow.com/preview/dcproject?utm_medium=preview_link&utm_source=dashboard&utm_content=dcproject&preview=925a45aabe67aa83287eec30790de4ff&mode=preview