Company software still on computer
There is no company software leftover on your iMac that is prompting you. The most likely explanation is that the iMac that you bought is enrolled into Apple Device Enrollment Program.

DEP lets organization automatically enroll devices into a MDM (mobile device management) system even without touching the device. The devices are enrolled into DEP at the time of purchasing via Apple or partner. Basically, Apple or resale partner associate the device serial numbers with the organization that made the purchase. This will let otganization IT team supply a configuration profile over the air, without even physically accessing the device.

The enrolled device checks with the Apple's servers to see if its enrolled into a DEP. You'll need to contact and ask the organization (basically the IT team thereof) to un-enroll your device from DEP. They should have ideally done this as a part of handing over the device to you (as it is no longer the property of the said organization).

Once un-enrolled, you'll no longer see the message and can use your Mac normally just like any other Mac.