Can IKEA FRAMTID built-in microwave be installed in a 24"-wide cabinet?
NO, you cannot install in a 24" cabinet. Minimum clearances stated in the installation instructions need to be followed or the manufacturer safety certifications will be null and void. If a fire starts because of improper installation, your insurance will not cover your claim either.

The "30" designation does not refer to cabinet size, it refers to the finished width across the front with the included trim pieces installed to cover the required 3" clearance on either side. The installation instructions will tell you to center the unit in the over-sized opening.