He's wrong, unless there is something very other than stock going on with your bike. There are three adjustments on the derailleur that apply here. The upper and lower stops, and the cable adjustment that allows it to shift smoothly through the various gears based on your selection via the shifter. There is also a chain tension adjustment, but that rarely needs to be messed with. When I set up a derailleur - and this can apply to tuning one as well - the first thing I do is set the lower stop (for the smallest, rear gear cog) such that the chain travels smoothly working the pedals forward and backwards. On our Levels, there is only one front gear cog, but if there were three - like my traditional bike - I would be doing this on the middle gear cog. I then fully tension the shifting cable to a point just shy of pulling the derailleur inward, and I shift upward to the highest - and largest - gear cog in the rear. Getting there may or may not involve adjusting the upper stop, but once there, I set the upper stop such that the pedals rotate forward and backwards with no issues, and boom. Job done. It sounds like your derailleur needs to be adjusted by someone who knows what they are doing. Mine needed adjustment when I got my Level two months ago. The upper and lower stops were set, but the travel between gears was not smooth, and that involved a simple cable adjustment. Plenty of YouTube vids out there regarding derailleur adjustments. Get it set up properly, and start having some fun riding..