I am currently trying to add AdsTerra ads to my Squarespace site, and I have the exact same problem.
I use Code Block, and make sure to uncheck 'Display Source". But the code never ever renders. I have tried code from AdsTerra for banner ads, native banners and social bar. No luck. I have tried putting the Code Block on different pages, at the top of pages, and at the bottom of pages, no luck!
I have checked with AdsTerra if I need to add any code injection in header or footer of my site, but no such requirement.
I have double checked that I am on a Business Plan - yes, I am. I have checked with Tech Support at Squarespace and at AdsTerra, no luck.
I have setup a test with a minimalistic "Hello World" javascript ona page, in a Code Block, to see if that would render, and it does.