Is steroid nerve block contradicted if sciatic nerve pain is caused by tumor compression? How long does the pain relief usually last?

Ask your Doctor. If epidural steroid injection is OK to do. Sometimes it is sometimes its too risky. Depends on your situation.
No it is not. There is no contraindication to steroid injection. But it is not very likely to help, if the tumor is causing compression of the nerve. It is better to try relieving the pressure by means of radiation therapy or some other means of causing shrinkage of tumor. Hyperthermia can be added to radiation as together they may work even better. You should seek 2nd opinion for further treatment of this tum.