When the approver launches Mekorma Power Approvals they get the following error message: "You are not a member of the environment security group."

Login as Admin to: https://admin.powerplatform.microsoft.com/environments  Navigate to the appropriate security group. Then, check if the approvers are listed as members of the group. If not, add the Mekorma Power Approvals approvers(s) as members.

For more information see Microsoft Learn:  https://learn.microsoft.com/en-us/power-platform/admin/control-user-access