In reply to Crusier.

That's correct. Support for ASA 5506-X, 5506W-X, 5506H-X, and 5512-X removed starting with FTD 6.3. The final supported FTD release for these platforms is 6.2.3. This is documented in the release notes: https://www.cisco.com/c/en/us/td/docs/security/firepower/630/relnotes/firepower-release-notes-630/new_features.html