cannot access specific team site using share-point rest apis

[buddhika_jayakodi]

goal: 

I want to create a folder in my custom team site in share-point using REST APIs.

prior steps I have done before that:

I have register the my app in share-point using following URL. https://***.sharepoint.com/sites/ModernTeamSite/_layouts/15/appregnew.aspx

In my App "permission request XML" , I have provide below content. 

<AppPermissionRequests AllowAppOnlyPolicy="true">
  <AppPermissionRequest Scope="http://sharepoint/content/sitecollection/web" Right="FullControl"/>
</AppPermissionRequests>

for get Access Token for accessing those REST APIs I have also created a app in "App Registration" under Azure portal.

when creating those I have provided following API permission for this app.

After doing so I was able to get the bearer access token.

Then I was able to create a folder, attache a file to a new app in share-point using rest apis.

but All those were happened in the "Communication site".

My requirement is to create a folder inside a custom site (team site).

for that I have created a custom site using web interface.

Name called: TeamSite_ForB

following is the REST endpoint url I'm trying to access:

https://***.sharepoint.com/sites/TeamSite_ForB/_api/web/folders

I'm using postman in this case.

so, in the authorization I have set the bearer token.

in the request header section I set the followings.

key	value
Content-Type	application/json;odata=verbose
Accept	application/json;odata=verbose
X-RequestDigest	0x4AAF05C94B5EB91BF70B45A41166FADD431D5613120E4D2741B9DF8B383807331D1158CD236CDBE8A1DDBFC2162620CE0D6A62586CDBE45DD01745D777C7D66B

in the request body I have set the following json body.

{ 
	"__metadata":{ 
		"type":"SP.Folder"
	},
	"ServerRelativeUrl":"/sites/TeamSite_ForB/Shared Documents/testFolder"
}

but when I send the request I'm getting following error response with "403 forbidden"

{
    "error": {
        "code": "-2147024891, System.UnauthorizedAccessException",
        "message": {
            "lang": "en-US",
            "value": "Access denied. You do not have permission to perform this action or access this resource."
        }
    }
}

**Important**

If I put url endpoint as below and request body as below I'm getting 201 response code and folder created inside "communication site".

https://***.sharepoint.com/_api/web/folders

request body

{ 
	"__metadata":{ 
		"type":"SP.Folder"
	},
	"ServerRelativeUrl":"/Shared Documents/testFolder"
}

so, I just don't want to create folder inside communication site, rather want to create inside custom team site. 

I googled many articles and go through the share-point documentation and I couldn't find any solution for this error.

please can anyone give me insight what I'm doing wrong here.

Any help would be highly appreciated.