I have a 2017 21.5 inch Mac and need to install more RAM. Is it possible? Don't see it in above list.

Yes, as explained in the very support document you posted from:

* 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), iMac (Retina 4K, 21.5-inch, 2017), and iMac (Retina 4K, 21.5-inch, 2019). If the memory in one of these computers needs repair service, contact an Apple Retail Store or Apple Authorized Service Provider. If you’d like to upgrade the memory in one of these models, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorized Service Provider offers memory upgrade services.

Memory in iMac (21.5-inch, Late 2015), and iMac (Retina 4K, 21.5-inch, Late 2015) is not upgradeable.

It does not have a slot to access the memory directly, so needs to be disassembled to access the RAM and can be upgraded by a qualified technician.

Who said anything about an iMac Pro? No such thing as a 21.5" iMac Pro

Did you mean: https://eshop.macsales.com/shop/memory/imac-2017-21.5-inch

Same caveat though. No memory access panel.

It can be done but see the following from https://eshop.macsales.com/shop/memory/imac-pro-2017

• Up to 512GB of OWC MaxRAM Certified Memory.
• Professional installation required.
• Difficulty level: Extremely difficult.

iMac Pro DOES NOT feature a memory door on the back allowing for easy upgrades. Upgrading iMac Pro memory requires dismantling the computer to gain access to the memory slots. This is a highly complex process that requires a skilled technician.

Phil0124 wrote:

Who said anything about an iMac Pro? No such thing as a 21.5" iMac Pro

The user marked iMac Pro in the signature. Even I could figure that out. 😎

LOL!, The 21.5" specification kind of overrides that. What to believe?

I'm going with 21.5". Of course you never know. 😉

dialabrain wrote:

I'm going with 21.5". Of course you never know. 😉

Indeed. 😉😜