The first reason is if you don't have enough probes to spy targets, the bot will wait forever.
If you do have probes, and the farm session terminate immediately, you probably ignored all the possible targets.
Open the modal to create a new session, and clear your "ignored lists".
If you are attacking with probes, and you get the "not enough cargo" message:
The bot will wait for all probes to come back to send the next attack. If you lost some probes since the beginning of the session, the bot will wait forever. Build more probes.