Pack Runs are a pack activity, therefore only one Pack Run Lobby may be open at a time. Another Pack Run may be hosted once the current Pack Run has started, allowing room for a new Run to be formed in the Lobby. Once the lobby is filled or the host starts the Run, another player can host a Pack Run.

If you see another Pack Run lobby open when you are trying to host a Pack Run, you will need to wait for that Run to start before hosting a new one.