Hi.
Adobe Stock does not support a refresh token at this time. It used to be allowed, but was taken away for security reasons because we cannot guarantee it will be treated securely. With any token, a user could use your account to do unauthorized licensing or even purchase Premium content. Net, any user of your application must sign in every 24 hours, or whenever the token expires.
If you are an Enterprise customer, you should be using a service account integration, which does not require a user login.
If you have further questions, please email stockapis@adobe.com.
Thanks,
Christopher