Hi @soumikiam,
This error message means that you haven’t successfully generated your access token, please try to authenticate again.
This issue occurs in the following cases, please check one by one to resolve the problem.
1. First of all please make sure that you have the latest version of Instagram Feed WD installed (that is 1.3.16 if free, 2.3.16 if pro).
2. The other case that issue occurs is when your website and/or Instagram app are in a foreign language (any other language than English).
If this is your case, then please make sure to switch both your website and Instagram app languages to English, get the access token then switch back to your language.
3. Please make sure that you are trying to connect to PUBLIC Instagram Business account.
4. Please enter WordPress dashboard > settings > general > site address please check this link and make sure that it is the same as your website link, mainly this is needed to make sure that in both places your URL starts with the same protocol, both should be either with HTTP or both start with HTTPs.
Please get back to me with results.