複合動詞N4
〜にくい
hard to do... / difficult to...
Structure
V-ます形（語幹）+ にくい
Explanation
Attached to the masu-stem of verbs, 〜にくい means something is hard to do (e.g., 読みにくい). Conjugates like an い-adjective.
Examples
この本は漢字が多くて読みにくいです。
This book has many kanji and is hard to read.
この靴は歩きにくいです。
These shoes are hard to walk in.
Note
〜にくい contrasts with 〜やすい. Note it means "hard to do", not "cannot do" — for inability use 〜できない.