Check your portal memory. You start with 20 memory, and each explore through your home portal uses up one memory. Memory also generates at a rate of one per hour or you can use Portal Flow Cards which substitute for memory. Alternatively you can use a roc with dominant Watchman for a single memory-free explore.

20 hours for 20 memory. There's a timer above the home portal that shows how much memory you have and when it will regenerate.