I have seen that in some sites they always keep " Search Button" in active / enabled state even if input fields are empty.
Is it a correct way or should we make it enable only when user selects / enter something.
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[...] simple search usually works best and should be presented as an input field with a submit button. Remember, the submit button is a button, which means it should be designed like one. In particular, the submit button should look different than the input field.
Users are conditioned to scan for the field + button. Disabling the button can trigger other consequences, such as users believing the search is not working at the moment. If users searching blank entries is a concern, there are measures to mitigate it such as (1) providing missing entry notification, (2) pushing trending categories or items in a new page, (3) offering a pathway to browse subpages via a sitemap page, etc.
If a user can start searching without any text input, they need to receive some meaningful results. It can actually be a feature: maybe your user doesn't know what to type in, but you think you have something to offer them anyway. Show them your hot offers, suggest something based on your knowledge about user.
There was a conception of "zen search" when you don't type anything, but get some random results after clicking [Search].
This is not a direct answer to your question, but maybe it can solve your problem. Your button can be always available and always working. No need to disable it. Just show people something good.