I installed Magento but in admin panel it is giving system message as:
One or more indexers are invalid. Make sure your Magento cron job
is running
If I am adding categories or products from admin panel It shows no effect on front-end and not even menu and categories are displayed.
Best Answer
You must configure a cron.job in your server to automate a some internal tasks like reindexing, email sending and cleaning between others, there is 3 cron jobs in magento2 you will find all information about that in:
Configure and run cron
Magento has a complex db schema to save categories and products data and "render" all these information to a single table per store if you have configured use flat categories and/or use flat catalog to grant performance on the frontend. This would be the reason why you don't see categories on frontend.
Remember always clean/flush cache too