There are a few different reasons why you may not be able to apply Mailbox Provider Weighting in your account. 

The most common reasons include the following: 

• You must have administrative rights in order to setup Mailbox Provider Weighting.
• The Mailbox Provider Weighting feature has not been turned on in the account configuration.
• Mailbox Provider Weighting values have not yet been set.
• Parent users need to set their own weight values before they can select Apply Mailbox Provider Weighting to apply to child accounts.