The Base64 validator checks whether the submitted text is a valid Base64 encoded string. It allows you to validate online a variety of Base64 standards. Therefore, try to specify another standard, if one of them failed.
If you do not know which standard to choose, check the standard detector. If you want to decode a string, use the Base64 decoder. If your string contains invalid characters or cannot pass the validation, use the Base64 Repair Tool.