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Dear Steffny Bill,
Thank you for providing the necessary information about the issue you are facing.
Kindly try the below troubleshoot and please share us your results with a follow up replay.
Please try creating a new a person or group column and also allow selection of groups. Check if it works on the new group new column.
Also please make sure you enter the security group name correctly. If you entered it wrongly, it might not be detected.
If you have any additional questions or if you need any additional help regarding this or anything else at all - let me know, I'll be here to help you out.
And in the meantime, I hope you'll have a lovely day ahead, stay safe & take care!
Yours sincerely