Indomethacin was one of the medicines studied together with valsartan in clinical trials. The leaflet does not describe any specific problem from combining the two. Valsartan is not broken down much by the body, so it is not expected to speed up or slow down the liver enzymes (the cytochrome P450 system) that process many other medicines. That means it is not expected to change how indomethacin is handled through that route.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.