What is the ruling on a husband who suffers from complete sexual impotence and is unable to fulfill his wife's rights, and resorts to imagining other women during foreplay, and is he sinful for lack of supplication despite feeling frustrated?

You must take the initiative to repent from going to sorcerers and increase your supplications for healing, for every illness has a cure. What you imagine regarding women is forbidden, but there is no sin upon you in remaining with your wife as long as she consents to it. Your feeling of pain and sadness does not negate your patience, unless you show discontent with Allah's decree. You have not incurred sin by supplicating little, but you have missed out on much good. So, trust yourself, stay away from obsessive thoughts, and aspire to true happiness in the Hereafter.