Hello,
Thank you for contacting us.
You problem is likely due to your hmac authentication code computation that is not correct.
For more details, please refer to these sections of the documentations:
https://developer.myscript.com/docs/interactive-ink/4.2/web/rest/architecture/#credentials
https://developer.myscript.com/support/account/registering-myscript-cloud/#identification-of-your-application
You can also look at this post of the forum that is related to the same topics.
Best regards,
Gwenaëlle
Farah. hosni