Search block uses WP Rest API to fetch search fields. Rest API does not work with a plain permalink structure. if you are seeing Loading.. text on the search block like in the screenshot below:
Please check for the permalink settings and consider updating the permalink settings to address this problem. To change the permalink setting from Plain to Post name, simply navigate to the Admin Dashboard > Settings > Permalinks.
Please look at the provided screenshot below for an illustration of where to make this adjustment.