“The Indus script (also known as the Harappan script) is a corpus of symbols produced by the Indus Valley Civilization during the Kot Diji and Mature Harappan periods between 3500 and 1900 BC. Most inscriptions are extremely short. It is not clear if these symbols constitute a script used to record a language, and the subject of whether the Indus symbols were a writing system is controversial. In spite of many attempts,it remains undecipherable, and no underlying language has been identified.” source: Wikipedia
Some scholars suggest that the scripts are not decipherable because they're not linguistic records (citing the extreme brevity of some of the entries). The average inscription uses just five signs, and the longest inscription is only 26 signs long.