Uploading a csv in box folder is giving 403

i'm new to scripting and was trying to upload a csv file from local to box folder. I followed code used here(https://support.box.com/hc/en-us/community/posts/360049140554-Upload-a-file-using-API-in-PowerShell) i was passing access token in the above code. But it was showing 403. I have also created a custom app in box which has following permissions:

And also added app in admins custom app manager.

Here is my code:

    #function that returns the content type based on the file extension
    function Get-ContentType
    {
    param([System.IO.FileInfo]$file = $null);
    $contentType = $null;
    $contentTypeMap = @{
".csv"= "text/csv";
    }
    
    if ( $file )
    {
    $contentType = $contentTypeMap[$file.Extension.ToLower()];
    }
    $contentType;
    }
    
    #uploads file to box
    function uploadFile {
    param(
    [System.IO.FileInfo] $filename = $null,
    [string] $folderid = $null
    );
    
    $boundary = [guid]::NewGuid().ToString()
    $contenttype = Get-ContentType -file $filename
    
    $headers = @{}
    $headers.Add("ServerHost", "https://upload.box.com")
    $headers.Add("Authorization", "Bearer $accessToken")
    $headers.Add("cache-control","no-cache")
    
    $filebody = [System.IO.File]::ReadAllBytes($filename)
    $enc = [System.Text.Encoding]::GetEncoding("utf-8")
    $filebodytemplate = $enc.GetString($filebody)
    
    #creating the formdata manually
    $template = "
    --$boundary
    Content-Disposition: form-data; name=""file""; filename="""+$filename.Name+"""
    
    "+$filebodytemplate+"
    --$boundary
    Content-Type:text/csv; charset=utf-8
    Content-Disposition: form-data; name=""metadata""
    
    {""name"":"""+$filename.Name+""",""parent"":{""id"":"""+$folderid+"""}}
    
    --$boundary--"
    
    #mail upload process beins with Invoke-RestMethod
    try{
    $upload_file = Invoke-RestMethod -Uri "https://upload.box.com/api/2.0/files/content" -Method POST -Headers $headers -Body $template -ContentType "multipart/form-data;boundary=$boundary" -Verbose
    
    
    if($upload_file.total_count -eq 1){
    $size = $upload_file.Entries.size
    write-host $upload_file.Entries.name " ($size bytes) uploaded successfully"
    }
    
    Write-Host $upload_file
    }
    
    catch [Exception] {
    Write-Host $_.Exception
    $exception = $_.ErrorDetails
    
    switch ($exception.status) {
    409 { Write-Host "name exists"}
    }
    }
    }
    
    $ClientId="<Client Id>"
    $Secret="<Client Secret>"
    $code="<Code from url>"
    $Uri    = "https://app.box.com/api/oauth2/token"
    
    $Body   = @{
            grant_type="authorization_code"
            client_id = $ClientId
            client_secret=$Secret
            code=$code
        }
    
    
    $resp = Invoke-RestMethod -Method Post -Uri $Uri -Body $Body -ContentType "application/x-www-form-urlencoded"
    
    #accesstoken
    $accessToken = $resp.access_token
    
    #provide file full path and the parent folder ID
    uploadFile -filename <path of file to be uploaded>  -folderid <folderId> 

I might be missing some other permissions.

Any help will be thankfull.