Magento 2 – Fix ‘One or More Indexers Are Invalid’ Error

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

1. Open Xampp control panel
2. Click on Shell button
3. Exec the command --> php C:\xampp\htdocs\XXXXXX\bin\magento indexer:reindex

Obs: XXXXXX is your site.