there is a problem with the build system of online tools for MbedOS 6.3 probably, please see this topic. And your board is supported only in the MbedOS 6.3.
Unfortunately for some reason the support does not respond and has been inactive for the last two weeks…
I have no idea but I hope for some support A.S.A.P.
But I see you are Mbed Partner so maybe your chances are better. Try to contact someone from Mbed team directly for some support.