Choose the policy which is in action (OwaMailboxPolicy in this example) and edit it.
Then, select the Outlook Web App policies tab in the top menu.Access your Exchange admin center and go to the permissions section.The most probable cause is OWA policy blocking this feature. This makes it impossible for users to make any changes to their Office 365 email signatures. When going through email settings, under Layout menu, the Signature button might be missing. Sometimes, when using your Office 365 Outlook on the web (OWA), you might experience a problem when trying to set an email signature for your account. Office 365 email signatures are not displayed in Sent Items.
Here is a quick access list of the most frequent email signature management problems, with solutions: In Office 365, there are some situations in which your email signatures stop working.
