Azure Powershell: Get-AzureKeyVaultSecret: Get operation denied

Question

I entered azure powershell using the certificate service principle, for example:

Add-AzureRmAccount -ServicePrincipal -CertificateThumbprint  $thumbprint -ApplicationId   $applicationid -TenantId $tenant

Then I tried to get the azure secret:

$b = Get-AzureKeyVaultSecret -VaultName $keyVaultName -Name 'AccountKey'

But I get an error message:

"get" is not allowed

It seemed to me that I was reading access, for example:

New-AzureRmRoleAssignment -RoleDefinitionName Reader -ServicePrincipalName $azureAdApplication.ApplicationId.Guid 

Any suggestions?