Hello @Nguyễn Quang Minh,
Welcome to Atlassian Community!
I see that you are adding a user through the People's page and it should work sending an invite for the user.
As the error doesn’t say anything about what happened, can you please open the developer tools on your browser and check on the Console tab if it shows any other error? If so, please send us a screenshot.
Can you also try to add the user through User management? Go to Cog Icon > User management > Users > Invite users and check if it works correctly.
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