Hello,
I think your code sets CFR1:4 (Clock Input Power Down) to 1, which brings the device in a deep power down mode.
In case this does not solve your problem: When I'm in doubt whether a DDS is running at all, I personally often use the Sync-CLK to drive out the System_CLK / 4 and measure that. That tests the communication and the system CLK setting all at once.
