Google Chrome Tab Search is Not Working

Go to chrome://chrome/settings/ then Manage Search Engines and in order for feature to work it must be added to the list of search engines. Chrome typically adds it automatically once you visit the site though.

In Chrome, visit chrome://settings/searchEngines
...You will see a list of all the tab-activated search engines, which Chrome has collected.

I will assume the search engine you want is NOT listed. If your engine IS listed [a problem I have NOT encountered], it may help to delete it, and re-add it using the following instructions:

Click [Add]

Search Engine: <Any friendly name you like>
Keyword      : <What you type in the URL bar to activate the search feature>
URL          : <The search URL with "%s" where your search words must appear>

Example ...I recently enabled the Youtube dark theme, which broke the Youtube "tab search" in Chrome:

Search Engine: Youtube
Keyword      : youtube
URL          : https://www.youtube.com/results?search_query=%s

Click [Add] (again)

If you do not know what the search URL should look like: Go to the website in question, perform a search, and look at what ends up in the URL bar ...Your text will be "URL Encoded", EG. spaces will be replaced by +'s (etc) - do not worry about that, the URL encoding will be done by Chrome's search engine for you :)

Example.
Go to Amazon and perform a search for "eneloop AA" which (for me) generates this URL:
https://www.amazon.com/s?k=eneloop+AA&ref=nb_sb_noss_1
...You might notice that if you remove the "&ref..." from the end, the search still works
https://www.amazon.com/s?k=eneloop+AA
...So our Search engine URL could be:
https://www.amazon.com/s?k=%s