Thank you for posting this in Microsoft Q&A.
I understand that you want to customize the error message in sign-in page.
To customize the error message in sign-in page you can use Custom CSS option https://video2.skills-academy.com/en-us/entra/fundamentals/reference-company-branding-css-template
Could you confirm whether you are attempting to customize the error message for External tenants or B2C tenants? If possible, please share the link you have been following so that I can assist you more effectively.
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.
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