My iMac has an Apple M1 chip. Can I install a Windows partition using Boot Camp Assistant?

No. Boot Camp isn't available for Apple Silicon Macs.

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You can get Parallels, which has an agreement with Microsoft to run Windows on Apple M1/M2 Macs.

Just to clarify, you can only run the ARM version of Windows 11 in Parallels 18 on an Apple Silicon Mac.