In the Order of the Phoenix, we see Dumbledore wants Harry to take extra lessons to learn Occlumency as a counter to Voldemort invading his mind, indicating that it is not something taught regularly to students his age at Hogwarts, and it seems that Harry's private lessons with Snape seem to be sort of secret
From what Snape says during the class, it probably is a breach of privacy to use legilimency on people, but it doesn't seem to be something out-right illegal such as the unforgivable curses, or highly regulated such as veritaserum
We also know from later scenes in that book that Umbridge is willing to take any steps to put Harry in his place and get information out of him (ready to use the Cruciatus curse on him in the later chapters)
If this is the case, why didn't Umbridge use legilimency against Harry during the scene when she was trying to find out where Sirius was, instead of using veritaserum? The veritaserum method ended up not working anyway, and I think from the later chapters it is revealed that it was in fact, fake.
Is there any plausible reason for why Umbridge did not use legilimency with Harry to find out the whereabouts of Sirius?