The reason for signature mismatch is that the one you created is different than the one server created. Check the following –
- Did you convert the signing key and base string into bytes before passing it to HMAC-SHA256 hashing algorithm. - Did you convert the output of HMAC-SHA256 hashing algorithm into base64 string.
also check this [link][1] if this can help you.

[1]: https://github.com/thephpleague/oauth2-client/tree/master/src

The reason for signature mismatch is that the one you created is different than the one server created. Check the following –
- Did you append ‘&’ at the end of access key secret to create signing key - Did you convert the signing key and base string into bytes before passing it to HMAC-SHA256 hashing algorithm - Did you convert the output of HMAC-SHA256 hashing algorithm into base64 string
Regarding error code, you can refer this - https://developer.here.com/documentation/authentication/dev_guide/topics/error-messages.html