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
-CertificateName
-AllowHttpSecureStoreConnections
-AllowHttp

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 a new farm using New-OfficeWebAppsFarm comment yesterday where I used "*.domainname.com” as a certificate name in this command, but I got an error that certificate was not found. Then today I used the friendly name of the 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 the Office web apps server.