Cannondale CAAD Optimo Tiagra 2020 CREAKING
return the bike to Cannondale as there is a welding fault in the area north of the front part of the frame

If true, that bike is probably not safe to ride.  If the shop showed you where the problem is, posting a picture of that area would probably be useful.  For what it's worth, if the shop that sold you the bicycle says it has an unsafe frame, I'd tend to believe them as that's an admission of a serious failure on their part, especially if they assembled the bike for you.
If the frame is faulty, Cannondale should warranty it no problem.

I can clearly hear the creak is coming from the crank area and appears to only creak when pedalling

That does not rule out frame issue causing the creaking sound.
First, it is extremely hard to isolate the actual location on a bike where a sound is coming.  And even then, because the components of the bike and and do conduct sound, where the sound appears to be coming from might not actually be the source of the sound.
Second, pedaling puts stresses on the bicycle that don't exist when you're just coasting.  Since significant power is applied to one pedal at a time, and the pedals are off-center, pedaling introduces twisting and sideways forces to pretty much the entire bicycle.  So a noise that is synchronized to your pedaling could be caused by something like a cracked front rim.
Finally, and completely independent of any frame weld issues, if the bike has a press-fit bottom bracket*, those are notorious for being noisy and a constant source of creaks and cracking noises.  This is a Google search for all the questions here about noisy press-fit bottom brackets: https://www.google.com/search?q=press+fit+bottom+bracket+noise+site%3Astackexchange.com
* - It likely does, but I can't confirm that from the product page as there doesn't seem to be a 2020 page but only a 2019 page that doesn't actually specify the bottom bracket type.