I have an issue with my bike, it works nicely until I am changing the gears. Every time I want to change one gear up, I have to stop pedaling or it won't change. There is no issue going back one gear.
Do you have the gears inside the hub? (I see city bike tag)
If so, the mechanism of the gear change requires that you ease the pressure from the pedals (or stop pedalling) or it won't be able to shift. That's normal!
If it is a derailleur system, heavily worn drivetrains or derailleurs with weakened springs can need that pause in your pedalling to help the shift (the extra time and lack of force through the chain can help the derailleur when it's at EOL) but it's a bad sign in this case, that some (major) overhaul is required.