Enabling the WordPress Search option on WordPress sites
There are two ways I know off to enable the WordPress Search option on WordPress sites that I know off:
Go into your WP dashboard, select appearance from the menu on the left hand side, then look for the search widget on the left hand side, you may have to page down until you see it. Then click on it, and add the widget to one of the options on the list. Make sure you actually click on the add button, otherwise the search widget will not be added.
Another way, and this is WordPress Theme specific. Sometimes a WordPress Theme will display a search bar near the top right of a screen, above where the right sidebar displays content or in a similar place. If so, this may be switched on or off.
Go into your WP dashboard, select appearance from the menu on the left hand side, select customize. Different WordPress Themes have the option in different places and use different text to describe it. I will explain how it looks in the WordPress Theme I currently use. Yours may be the same or not.
After selecting customize, there is a menu on the left hand side. One of the options is called "Header Settings" or something similar. One of the options in here is "Display Search Form" or something similar. Enable it if it is not currently enabled. Make sure you actually publish the changes, otherwise whatever you changed will not be active.
Remember that different WordPress Themes may have this search option in customize and that it may be in a different option inside of customize for the WordPress Theme that you are currently using. In this case, look at the list of options, and see if you can determine where it might be.
Otherwise you will need to go through each option on the customize menu to look for it. Should not take you more than 5 - 10 minutes to look through each one anyway, and you may find additional useful options. And the text "Display Search Form" may be different as well.