[pinkpanther]

The idea that Satan, or Iblis in Islamic tradition, cannot harm all prophets is rooted in theological beliefs across various religions, including Islam and Christianity..

I want to know why a true prophet like Muhammad was influenced by Satan
The most notable account comes from the works of Islamic historians and commentators, particularly Al-Tabari and some others. The story suggests that Muhammad later renounced these verses, claiming that they were inspired by Satan rather than by God. 

In the Hadith, it was also reported that Muhammad was under a spell or black magic for 6 months and more.....a true prophet will not succumb to Satan and black magic..why did Allah not help him?

Narrated Aisha:

Magic was worked on Allah's Messenger (ﷺ) so that he used to think that he had sexual relations with his wives while he actually had not (Sufyan said: That is the hardest kind of magic as it has such an effect). 

https://sunnah.com/bukhari:5765