Harappan writing is difficult to decipher because the longest text only has 24 characters and examples of Harrapan writing were still fairly limited in 1989. None of the near neighbors of the Harappan civilization were literate and no bi-lingual texts have been found that would help to crack the Harappan writing.
Note, the information comes from a short chapter on the Harappan civilization in the 1989 book "Civilization before Greece and Rome" by HWF Saggs.

As others have written, it's a combination of: *Very few inscriptions, most of which are short. *No bilingual inscriptions. *No knowledge of the language they represented, though it may have been Dravidian.

Where is this writing/culture from?