How authentic is the Hadith: "Whoever invokes blessings upon me two hundred times on Friday, Allah will forgive him the sins of two hundred years," and how can forgiveness be for two hundred years when a person may not live to that age?

The mentioned report has no basis, and we are not aware of any origin for it. It appears to be a fabricated lie attributed to the Prophet, may Allah’s peace and blessings be upon him. It has been authentically reported from him, may Allah’s peace and blessings be upon him, that he encouraged invoking blessings upon him, saying: "Whoever invokes blessings upon me once, Allah will invoke blessings upon him ten times." And in some narrations: "And Allah will record for him ten good deeds and erase from him ten bad deeds." And Allah, the Almighty, says: "Indeed, Allah and His angels send blessings upon the Prophet. O you who have believed, ask [Allah to confer] blessing upon him and greet him with [profound] salutation." (Al-Ahzab/56).