Uploading Files Via API from Salesforce

I am new to Box . I need to upload files to box via Salesforce.
I googled and found a link to do it:
https://techdicer.com/salesforce-and-box-integration-using-apex/ 

Consider I need only uploadFiles method .
As per the link , I changed client key , client secret and BoxSubjectId(Enterprise ID)
First I tried to upload 1 file under another folder but I was getting StatusCode 400 .

Then I tried to upload file to the root folder itself by setting parent_id as 0
I am receiving Success code 201 , but the file is not showing in Box
Even while trying to upload the same file again , I am getting error saying the file is already present , but still I can't view the files

Help and Concerns are welcomed!!
Thanks In Advance!!!