Guide to Fix The Menu Not Showing Issue

Log in as an administrator and go to the Admin Dashboard.
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Go to System Settings > Module Management > Curly Bracket Tags

Look for Menus under the column Name and click Edit

On the line code #if (node.Children.Any()) from the children menus add the data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false" inside the < a > code shown on the image below

Test the mobile menu