i need help

Technically the 4K models can be upgraded by repair technicians. However it is a difficult procedure that is not offered by every authorized service provider. https://support.apple.com/en-us/HT201191#note

If you wish to perform ram upgrades yourself then returning it and getting a 5K model is the way to go.

The 21" haven't had accessible RAM slots for several years. So, you have to buy it with all the RAM you want when you order it.

As long as you are within 14 days, you can return it with no questions asked. And then order a 27" (or order a 21" with whatever RAM you need. Also: stay away from snail speed slow 5400 rpm spinning drives; the 7200 rpm drives in 27" are ok, but if you can afford it, go with an all SSD drive - the speed is amazing.

what model is it?

if it is a 21.5", it cannot be upgraded.

if it is a 27", you can do it yourself and save considerably. Check either OWC www.macsales.com

or Crucial for compatible RAM modules.

Where, exactly, did you buy it? I can't imagine an actual Apple employee making that mistake.