One of the most common Microsoft Outlook issues that occur with most Outlook clients is the search tools not working as it should. Microsoft Outlook Search function not working is an issue that is common across all versions because all of them have the same base on how it functions, and that is the Microsoft Outlook Search Index. There’s no need to worry though, there are things that will help you get your Outlook 2017, 2016, 2013, 2006, 2007 search up and running again.
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If you're experiencing issues with the search function not working in Microsoft Outlook, you can try the following steps to fix the problem:

Check Outlook Indexing Status: Outlook relies on an indexing feature to enable quick and efficient searches. If indexing is incomplete or not working properly, it can cause search issues. To check the indexing status, follow these steps:
a. In Outlook, go to the "File" tab in the top left corner.
b. Click on "Options" from the drop-down menu.
c. In the Options window, click on "Search" from the left-hand menu.
d. Click on "Indexing Options" button.
e. In the Indexing Options window, check the status of Microsoft Outlook. If it shows as "Indexing in progress," wait for it to complete. If it shows as "Indexing is incomplete," click on the "Advanced" button, and then click on "Rebuild" to start the indexing process again. This may take some time depending on the size of your mailbox.

Check Search Settings in Outlook: Make sure that the search settings in Outlook are configured correctly. To do this, follow these steps:
a. In Outlook, go to the "File" tab in the top left corner.
b. Click on "Options" from the drop-down menu.
c. In the Options window, click on "Search" from the left-hand menu.
d. Under the "Results" section, ensure that "All folders" is selected.
e. Check other settings, such as "Include results only from" and "When searching, show results from" to make sure they are configured according to your preference.

Rebuild Outlook Profile: If the search issue persists, you can try rebuilding your Outlook profile. Follow these steps:
a. Close Outlook.
b. Go to the Control Panel on your computer.
c. Click on "Mail" or "Mail (32-bit)" to open the Mail Setup window.
d. Click on "Show Profiles" button.
e. Select your Outlook profile and click on "Remove" to delete it.
f. Click on "Add" to create a new profile, and follow the on-screen instructions to set up your Outlook account again.
g. Open Outlook with the newly created profile and check if the search function is working properly.

Repair Office Installation: If none of the above steps work, you can try repairing your Office installation, as it may be related to a corrupted installation. To repair Office, follow these steps:
a. Close all Office applications, including Outlook.
b. Go to the Control Panel on your computer.
c. Click on "Programs" or "Programs and Features."
d. Find your Office installation in the list of installed programs and select it.
e. Click on "Change" or "Modify."
f. Choose the "Repair" option and follow the on-screen instructions to repair your Office installation.
g. Once the repair process is complete, restart your computer and open Outlook to check if the search function is working correctly.

Hope You Find This Useful,
Peter