How can I import issues from CSV file when Jira is not recognizing assigned users to these projects?

[Jazmin Garcia]

Jira is not recognizing one of the users when I try to import issues/projects from a CSV file. It gives me the following error code:

Issue can't be created due to the error/s [ User 'Jira_Core' was not found in the system. ] 

The other users currently in the system use the username before the @ in their email. We are not sure what is going on this time 

[Brant Schroeder]

@Jazmin Garcia 

Welcome to the community.  Does the user in the CSV have an account in your instance?   If so you can try using their email address in place of the username and see if that works.  You can also export the user to confirm the username https://confluence.atlassian.com/cloud/export-users-833925313.html