There are certain restrictions that can prevent you from placing a creature in a Sanctuary. The first major one being you cannot place more than four creatures in total across all your Sanctuaries. You can either place all four creatures in one Sanctuary or spread them across multiple Sanctuaries. 

 

Additionally, the following creatures cannot be placed:

• Creatures that are on your Strike Team.
• Hybrid creatures.
• Creatures that you already placed in another Sanctuary (creatures can’t be in two places at once). 
• Creatures that are too dangerous for your current Player Level, as shown on their creature card.