right at the top, if you scroll past “27-inch” you’ll see this gem:
“*Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2017). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider.”
@vbezhenar posted a link to Apple Support in his comment: https://support.apple.com/en-us/HT201191
right at the top, if you scroll past “27-inch” you’ll see this gem: “*Memory is not removable by users on iMac (21.5-inch, Late 2012), iMac (21.5-inch, Late 2013), iMac (21.5-inch, Mid 2014), iMac (21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2017). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider.”
i call this the “iMac pitfall”.