Error 1046 can appear during HMRC verification due to several reasons, including incorrect user details entry.

Ensure that all details entered are correct. If acting as an agent, ensure there is a tick box selected for authorising the agent to submit requests on behalf of the employer. The credentials can be reentered within the Employer tab under the 'RTI  submission' section.

Once the details are entered correctly within this section and the user is set up correctly on HMRC, there should be no error appearing on the submissions.