Hi, good day.

The search at the side bar doesn't work for entering less than 4 characters for searching, it returned an error message saying record not found but it is exist. Can any expert help where can I rectify this problem? Thank you.

I have tried with Bender, Sigma, Modern themes but all having the same problem.

Its restriction from database.
Try to check this article:

https://docs.osclasspoint.com/fine-tuning-mysql-full-text-search-improving-search