Can we use betadine scrub on rashes of scalp?

Yes. Yes you can but you should see your dermatologist for treatment and evaluation to get the correct diagnosis and treatment.
Rashes. Their are a number of ointment, cream, solutions and shampoo for the scalp, you first need to identify the bacteria, fungus or lice , then apply the right solution, each treatment is different and the frequency of applications are also different.