The error message you are encountering regarding insufficient storage when copying files to Personal Vault in OneDrive is likely due to the limitations imposed on free users. While free OneDrive accounts have a certain amount of storage, the Personal Vault feature has additional restrictions that may not be clearly communicated.
For free users, the Personal Vault is designed to provide an extra layer of security for sensitive files, but it may have a lower storage limit compared to the overall OneDrive storage available to you. This can lead to situations where you can copy some files to the Personal Vault but encounter errors when trying to add more, especially if you are approaching the limit set for that specific feature.
If you require more storage for your Personal Vault, you might consider upgrading to a paid subscription, which would typically increase your storage limits and potentially remove these restrictions. However, it's understandable that the lack of clear communication regarding these limitations can be frustrating.