If you are running macOS 10.14, there are new security features that have been implemented. HTML Email Signature requires permissions to interact with MS Outlook to insert the signature. If you inadvertently disallowed this on the first attempt, you can reauthorize as follows:

1. From System Preferences, choose 'Security & Privacy'

2. Choose 'Automation' from the left-hand dropdown

3. Tick the box to allow MS Outlook on the right side (you may have to click the lock on the bottom to authorize changes)