Ram upgrade for iMac (Retina 4K, 21.5-inch, Late 2015)

Hey! My mac has been running really slow lately, so I‘ve been thinking about upgrading my mac. When I look into it on the page for ram-upgrading, I can‘t seem to find my mac. Does that mean that I can‘t upgrade the ram on my own? When I look into it, my iMac says that currently 8 gb are installed, with an option for 16 gb. Could you tell me what that means? I have a iMac (Retina 4K, 21.5-inch, Late 2015)

Thank you very much!