Hi @Paul, Avishek ,
Welcome to our forum!
Based on my research, i found the below information mentioned in this official article: Protect your OneDrive files in Personal Vault.
If you don't have an Microsoft 365 Family or Personal subscription, you can only add up to three files in your Personal Vault. If you do have a subscription, you can add as many files as you want (up to your storage limit).
Please check your subscription and confirm it is *Microsoft 365 Family or Personal subscription.
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