DocuSign Demo: Error "This authentication mode is not supported: NONE" after signing a document

We will soon start a project using DocuSign APIs from Salesforce.
Currently running some proof of concept to test signature with sample document.

The JWT authentication works well but when trying to create an enveloppe to perform a test signature with Demo Docusign org, getting error : "This authentication mode is not supported: NONE"
This error occurred both when recipient is an external or an internal docusign user email address.

Followed this documentation : https://developers.docusign.com/docs/esign-rest-api/reference/envelopes/envelopes/create/

Here's the sample request sent :

{
	"emailSubject": "Please sign this document",
	"documents": [
		{
			"documentBase64": "dGVzdCBkb2M=",
			"name": "Lorem Ipsum",
			"fileExtension": "txt",
			"documentId": "1"
		}
	],
	"recipients": {
		"signers": [
			{
				"email": "email@add.com",
				"name": "Contact Name",
				"recipientId": "1",
				"tabs": {
					"signHereTabs": [
						{
							"documentId": "1",
							"name": "SignHereTab",
							"pageNumber": "1",
							"recipientId": "1",
							"tabLabel": "SignHereTab",
							"xPosition": "75",
							"yPosition": "572"
						}
					]
				}
			}
		]
	},
	"status": "sent"
}

Any idea what can be the cause of the authentication mode error ?

Answers provided in this similar question did not help:
https://stackoverflow.com/questions/72966631/docusign-this-authentication-mode-is-not-supported-none-when-trying-to-finis

Tried to add a signatureProviderName but webservice request itself fails

"recipientSignatureProviders": [{ "signatureProviderName": "UniversalSignaturePen_ImageOnly" }] 

Error : The signature provider name specified is not valid. Invalid signature provider name: UniversalSignaturePen_ImageOnly

Adding identityVerification via phone number in request eliminates the error, but we were told that phone sms verification should not be mandatory in all requests.

Does your account have the global setting for requiring all envelope recipients to be authenticated enabled? That would then flow over to API calls as well. Check the "Security Settings" in your Admin settings page for this and if it is enabled, you'd need to disable it.

Following a case logged with DocuSign support, the root cause of the error is electronic seal is enabled on the account and no digital signatures are enabled.

Here's response from support:

> This error is occurring due to the fact electronic seal is enabled on the account and no digital signatures are enabled. To resolve this issue, I will need to enable the DS Electronic signature type on the account. As this will be the only signature available on this account, it will be applied by default to all envelopes sent from this account. Users won't need to select it when creating envelopes.

After support enabled DS Electronic signature type in the internal settings, this resolved the error and no need to send identityVerification when creating enveloppe via APIs