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Dear Yul_N,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, it seems that you are having issue with access request in SharePoint for external users.
If the understanding above is right, based on the error message you're getting, please check if changing to another browser and different network can help. Please also check if the suggestions shared in below official article can help.
For your reference: Request approval failed when you process a pending request - SharePoint | Microsoft Learn
If the issue persists, considering that you are already "part of the group with full owner access", issue could be related to your account or the tenant background service which need to be checked from the backend. As forum support, we may not have enough permission and resource to directly check the backend environment, more resources need to be involved in to help you further.
In this situation, we sincerely recommend you contact your Microsoft 365 administrator, follow steps in this article to create a support ticket with Online Support. The support team over there has higher permission than us, they can remotely help check the situation on your side, involve more resource to collect logs and help you do troubleshoot. This would be a more efficient way in handling this case for you.
For reference: How do I find my Microsoft 365 admin?
Sorry for the inconvenience again. Your patience and understanding are highly appreciated. Hope you stay safe and healthy.
Sincerely,
Rhoda | Microsoft Community Moderator