Cables don't stretch. The cables and casings bed in. Sometimes the cable ends don't engage fully into the cable stops when the bike is on the shop stand, but "snap" into place on first actual use.

Loosened cranks on square tapers (even for a short period of riding) are at risk of on-going problems that can wreck the cranks, especially the left-hand one.

Mr Gee, it's a simple adjustment of the barrel adjuster. Take it back to the LBS and get them to do the job they are obligated to do, if it is a brand new bike.

Otherwise, if you want to do it yourself, put the bike on a stand, move the derailleur to the smallest cog. Check the amount of slack in the cable on the downtube. That is your starting point... if it is obviously excessive, use the anchor bolt to make it taut.

Then turn the cranks and shift your way up the gear and use the barrel adjuster at the back of the derailleur for fine adjustment. Sight along the chain from behind the cassette, and there should be a small amount of clearance on the right side of the chain in every gear except the big cog (obviously). The teeth on the cog should be centred on the chain.

I endorse also checking that the lockring is tight, but usually looseness results in a tingling sound as you ride.

People are too quick to denigrate "low-level" components for human error in setting them up.