Is this ram compatible with iMac 27"?
General
You selected a DDR4-Generation module in the implementation of SO-DIM which is both matching.
But I am not quite sure about the frequency. DDR4 has two standard frequencies: 2400 and 2666 MHz. Normally 2400 is always supported as default, and the 2666 is optional for higher performance.
However Apple is saying you should use a 2666 MHz for the iMac Retina 5K 27-inch 2019 model.
So you should use a memory module that is a Unbuffered Nonparity DDR4 SDRAM 2666MHz (PC4-21333) SO-DIMM. The brand should not matter.
For detailed information what all that means take a look at wikipedia.

Answer:
You selected a 2400 MHz module which normally should work, but apple seams to recommend 2666 MHz. To be 100% sure buy a 2666.
Suggestion:
You selected a Crucial module, on their web site you can search for your system and list all memory modules that are compatible. Here you can only find 2666 MHz modules. So if you want to stick to crucial you can follow this link. That would be my suggestion because the modules match the Apple specificities and are also sold as compatible for your specific system.

Apple Information
On the web site you can view the specifications for the RAM modules.
Memory specifications (iMac Retina 5K, 27-inch, 2019)
This iMac model feature Synchronous Dynamic Random-Access Memory (SDRAM) slots near the bottom of the computer with these memory specifications:
Number of memory slots: 4
Base memory:    8GB (2 x 4GB DIMMs)
Maximum memory: 64GB (4 x 16GB DIMMs)
Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:
PC4-21333
Unbuffered
Nonparity
260-pin
2666MHz DDR4 SDRAM