namespace attribute must be a simple Javascript name.

Invalid value specified for the namespace attribute: fmfctest. Must be a simple Javascript name.

[britishboyindc]

I got this today - I hadn't linked my namespace to my devhub via the name space registries

[KamilSzostak]

I got the same error when my namespace was not linked to the DevHub.

Linking a Namespace to a Dev hub solved the problem.

[mattlacey]

@wadewegner Any chance we could get a more descriptive error in this scenario? I just hit the same issue as Peter and Kamil, just had to link up the namespace in the registry but would never have guessed that from the message!

[KeithClarke]

+1 for including not having the namespace linked as a possible cause in the error message.

[wkuehler]

Just ran into this error as well. Thanks @KamilSzostak for the namespace tip. Agree that the messaging could be better here.

[kokasvmx]

I have encountered the same, this thread helped me to resolve. One other step to be performed once namespace registry is linked is I have reinitiated the force:auth:web:login to my DevHub org