See https://www.math.columbia.edu/~woit/wordpress/?p=10560, which links to a technical write-up by Scholze and Stix about what they think is the issue with Mochizuki's proof.
Woit's blogpost also gives a bit more links, including a response by Mochizuki.
The point in the proof where scholze and stix show it fails is “IUTT-3, Corollary 3.12”