Run the following command to set Outlook Web App enabled for all user mailboxes
Get-Mailbox -RecipientTypeDetails UserMailbox -resultsize unlimited | Set-CASMailbox -OWAEnabled $true
Run the following command to set Outlook Web App enabled for all user mailboxes
Get-Mailbox -RecipientTypeDetails UserMailbox -resultsize unlimited | Set-CASMailbox -OWAEnabled $true