I am connecting my T-Tap to a 2018 MacBook Pro with a Thunderbolt 3 to Thunderbolt 2 adapter. AJA Control Panel says 'Unsupported AJA Device' and I don't get video out of Avid. I tried uninstalling and re-installing the AJA driver. I'm running High Sierra 10.13.6 and Media Composer 2018.9.
I think the T-Tap is maybe a Thunderbolt 1 device? But I read that the adapter I have will work with Thunderbolt 1.
I am using a T-tap with an iMac Pro. Coming out of the Thunderbolt 3 port, into an Apple Thunderbolt 3 to Thunderbolt 2 adapter, then into the T-tap. Loaded the latest AJA software and the T-Tap gives me video out of both the SDI and the HDMI without any issues. It sounds like you need that Thunderbolt3 - to Thunderbolt 2 cable.