A systematic literature review is needed in the topic of “Challenges of Explainable Artificial Intelligence (XAI)”. There are lots of resources about Explainable Articial Intelligence. it is needed to review and analyze the challenges of XAI and prepare a systematic literature review about this. I will add some papers. And some more References: 
Adadi, A., & Berrada, M. (2018). Peeking inside the black-box: A survey on explainable artificial intelligence (XAI). IEEE Access, 6,
52138–52160.
Gilpin, L. H., Bau, D., Yuan, B. Z., Bajwa, A., Specter, M., & Kagal, L. (2018). Explaining explanations: An overview of interpretability of
machine learning. In 2018 IEEE 5th International Conference on data science and advanced analytics (DSAA). IEEE (pp. 80–89).
Adadi A, Berrada M (2018) Peeking inside the black-box: a survey on explainable artificial intelligence
(XAI). IEEE Access 6:52138?52160 
Arrieta AB, Díaz-Rodríguez N, Del Ser J, Bennetot A, Tabik S, Barbado A, García S, Gil-López S, Molina
D, Benjamins R et al (2020) Explainable artificial intelligence (XAI): concepts, taxonomies, opportunities
and challenges toward responsible AI. Inf Fusion 58:82?115