Q: Where can I find Sandbox Credentials?
and personal testing credentials for your store
Personal test credentials
Q: Why is my hash failing?
It is highly recommended to copy and paste e.g. the secret key through for example Notepad or some other plain text editor. It is common problem that the hash calculation fails simply because there are invisible whitespaces or line feeds / carriage returns in the value when it has been copied from rich text editors (e.g. MS Word) or email clients (e.g. MS Outlook).