The sign-up prompt appears because your end users don’t have the right Power BI license or permissions in the Power BI Service.
What to do:
Licensing – Either assign users a Power BI Pro license or place the report in a Premium capacity workspace (Premium allows free users to view).
Permissions – In Power BI Service, share the report or dataset with an Azure AD security group containing all viewers.
Embedding – Use Embed in SharePoint Online from Power BI and paste the link into the Power BI web part in SharePoint (not Publish to Web).
Reference guide with step-by-step instructions here: Embedding Power BI Reports in SharePoint – Step-by-Step
Reference:https://learn.microsoft.com/en-us/power-bi/collaborate-share/service-embed-report-spo