Balso already mentioned two of the places Google AI provided.
El Mirador de Campos: Located just 15 minutes south of Palencia in Autilla del Pino, this elevated escarpment gives a sweeping, unobstructed view of the western meseta. It is highly rated and serves as a premier stargazing and sunset vantage point
Las Tuerces: A surreal limestone karst landscape in northern Palencia. While situated further into the province, visitors frequently note it provides one of the most stunning sunset and western horizon views in the region, with the river and valleys stretching below the rocks.
Cristo del Otero: Positioned on the northern edge of Palencia city, this iconic 21-meter statue sits atop a tall hill. It is the closest major scenic point to the downtown area and provides a spectacular downward and horizontal sweep of the horizon.
Mazariegos: This rural municipality situated directly within the flat Tierra de Campos plains features wide open, perfectly flat western skies that are ideal for chasing low-horizon sunsets or observing astronomical events.