If you use MGS theme then install Pop Up for Magento 2 extension but don't see it work properly, seemingly the reason is our module css hasn't been generated yet.

We would like to provide a solution that:

Step 1: create a new file: l/app/design/frontend/Mgs/claue/web/css/third-party_extension/_Bss_Popup.less  

Step 2: import css to your theme by adding code into this file /app/design/frontend/Mgs/claue/web/css/theme.less 

@import 'third-party_extension/_Bss_Popup.less';

If you still couldn't fix the issue, please feel free to contact us for further support.