This happened to me the other day.
So how to troubleshoot… Look in the ULS logs.
To configure office web apps you’ll need:
-Office Web Apps Farm Internal URL:
-Office Web Apps Farm External URL
For me it turned out to be the certificate.
I found errors in Office Web Apps ULS logs, so I had also run Get-OfficeWebAppsFarm command which showed me the same error that the computer is not part of an Office Web Apps Server Farm. So I tried creating new farm using New-OfficeWebAppsFarm comment yesterday where I used “*.domainname.com” as certificate name in this command, but I got error that certificate was not found. Then today I used the friendly name of certificate and it worked.
New-OfficeWebAppsFarm -InternalUrl “https://sharewap.greenpages.com” -ExternalUrl “https://sharewap.domainname.com” -CertificateName ” domainnameDigiCert Wildcard” -EditingEnabled
This problem usually happens due to windows updates getting installed on Office web apps server.