Most cases of obstruction of justice arise from the intent to interfere with either a judicial or Congressional proceeding. Usual cases of obstruction of justice stem from evidence tampering, witness intimidation, or failing to report a crime.
Attempting to obstruct justice is enough to result in a conviction for obstruction of justice. Whether you successfully obstructed justice or not doesn’t matter.
Even if obstructing justice was impossible, attempting to do so can result in a conviction.
Obstruction of justice is defined in 18 U.S. Code Chapter 73.
Obstruction of proceedings before departments, agencies, or committees is described in 18 U.S. Code Section 1505. Section 1505 covers obstruction of an FBI investigation.
This page from the Department of Justice gives a lot of information about Section 1505.