I've been trying the following in Postman:
GET https://{{mycloudinstance}}/rest/api/2/mypreferences --user foo@bar.com:{{myjiratoken}}
and always get the response:
{ "errorMessage": "Site temporarily unavailable"}
I'm logged into the site so I'm pretty sure it's available.
I saw somewhere on the developer forum that you need to use a different base URL when using a token, but that discussion was for Confluence and I can't find a similar comment for Jira.