There are a couple of Taskbar settings to check if you use multiple monitors, moved your Taskbar, or use a smaller Taskbar.
Click on Start > Settings > Personalization > Taskbar.
If you're using a small Taskbar, the Search box won't appear. Uncheck Use small taskbar buttons
If you're using multiple monitors, make sure Show Taskbar on all displays is on. Speaking of multiple monitors, head to System > Display and click Identify. Pick, or confirm, the correct monitor is the main display.
Finally, if you've moved your Taskbar, the search box is only available when the Taskbar is at the bottom. Using Step 1, enable the Search icon.
3: Windows 10 Search Missing - System File Checker