Thank you for posting this in Microsoft Q&A.
To invite external users into your tenant you need Guest Inviter role or a User Administrator or a Global Administrator. Can you please confirm which role you have
I have tried from my end and successfully invited an external user.
Could you please try using another browser from your end? You can also use PowerShell to send the invitation Invite guest users with PowerShell
If you are still facing issues, please capture and share the audit logs of the activity.
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya.
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