He is claiming an advanced civilization that spanned the globe existed at the end of the last Ice Age around 12 thousand years ago. This is not millions of years ago. This is very recently in geologic time.

Such a civilization would have been at least as advanced as our 16th century if not more advanced. For sure, it would have left something, including structures, or even its trash. We can find stone tools from that era all over the place, but we can't any evidence for an advanced civilization that was basically as advanced as ours.

How convenient that it only existed on the coasts, in exactly the same areas that are now underwater. And it's not like we haven't explored those areas. People find shipwrecks all the time. And archaelogists and paleontologists are actually surveying some of these areas. For example Doggerland, which is the submerged area around the UK. They found animal bones, a Neandertal skull, even stone tools. They haven't found anything advanced. You can check out more about these underwater investigations here:

https://lostfrontiers.teamapp.com/?_webpage=v1

and here:

https://www.bbc.com/news/uk-england-norfolk-48594701

Gobekli Tepe or ancient megaliths are not that advanced. They could have easily been built by the locals and their stone age cultures.