As the Octamic II has not adat inputs you have two options for clock synch

a) to use Word Clock
b) to configure Octamic II as clock master (clock source internal).
For this to work you need to configure the Clarett as clock slave (to get clock from it's ADAT input).

If you want to test the ADAT cable plug a loop at the Clarett and check whether you get lock and finally synch status.

What Clarett do you own? Please send a link to that product.

What is your sample rate? 44.1 kHz?

Check DIP Switch of Octamic II, all DIP switches in the position "UP" for 44.1 kHz and internal clock (master).

Ensure that you connect the correct ports on both sides:
- Main ADAT output of Octamic II (the one labeled MAIN)
- to MAIN ADAT input of Clarett (if there were two)

How do you actually test the connection or what do you have connected to the preamp? Which mic, what about the level meter on the Octamic II? Do you see a signal (green LED Sig). Gain turned up? Is phantom power +48V required?
Did you check with another Micro or can you test it on the Clarett with the same cable?

If it still doesn't work. Could you post a photo of both backs, the front of Octamic II showing Gain knob and signal and screenshot of Focusrite Mixcontrol for routing or other interesting settings in that context?

You're welcome, well, this was it, wrong sample rate.

Maybe a better idea to use Word Clock:
- Make your Focusrite Clarett to clock master (clock = internal)
- Configure Octamic as clock slave, getting the clock from WC

For the Octamic II, you would still need to set the correct sample rate by DIP switches.
But the big advantage that I see: for your monitoring at the Clarett the sample rate would always be correct
and the correct sample rate would be set by the (Focusrite) ASIO-/WDM-driver.