Will you please explain a little bit more about your use case and how you have it implemented so far?
If you're expecting a standard Salesforce user to trigger your apex to send a DocuSign envelope transaction, then your standard user needs the "DocuSign Sender" permission set assignment assigned to them, not "DocuSign User". But would need to know more about your use case and requirements to provide further assistance.
Here's more documentation that might help you:
Understanding DocuSign Permission Sets in Salesforce: https://support.docusign.com/s/document-item?language=en_US&bundleId=srl1586134368658&topicId=xxm1623778416524.html&_LANG=enus
Enable a System Sender To Let Non-DocuSign Users Send and Sign Envelopes With the Apex Toolkit: https://support.docusign.com/s/document-item?language=en_US&bundleId=srl1586134368658&topicId=jxz1611642321527.html&_LANG=enus