If a shop did it, take it back and tell him he worked on it and now there is an error message. The error message needs to go away. It can be turned off in the bios, or as mentioned above one of the rad fans can be plugged into the CPU fan header. I'm sorry we can't be 100% specific. We don't know which B550 board you have, or which Rad and fans you have.
You should restart your computer and enter the BIOS or UEFI settings. You can usually access these settings by pressing a specific key during the boot process, such as F2, Del, or Esc. The specific key may vary depending on your computer's manufacturer.
Do you at least know what make and model motherboard and CPU liquid cooling you have?
And in my opinion, that guy in the "reputable shop" is at least partly wrong. You very much need a pump warning since a pump failure is more critical to liquid cooling than a fan failure is to air cooling. The fan speed sensor can be used to monitor pump speed in the same way when connected properly.
If what he installed is an AIO he should connect the pump to the CPU fan header, adjust it for maximum speed all the time and then monitor the pump (fan speed) to alarm if speed falls below a critical value. Many if not most B550 motherboards allow for this. He might have gone the easy way and connected it to a SATA power connector where it's not monitored at all.