Reading the FTD 6.3 release notes it states that:
You cannot install Firepower Threat Defense 6.3 or subsequent releases on the ASA 5506-X, 5506W-X, 5506H-X, and 5512-X. The final supported FTD release for these platforms is 6.2.3
This is incredibly disappointing as I have a number of customers utilising 5506-X most of those purchased in the past 12 months, and heavily utilising the firepower features.
