If you pause a study and then receive an error saying "insufficient funds" when trying to re-launch it, it's likely because of the way MTurk holds funds to cover outstanding charges.
When you pause an MTurk Toolkit study, MTurk prevents new workers from entering the study, but workers who have already started the study are given time to finish. MTurk does not terminate workers if they accepted the HIT just before you paused it or if they are currently working on the HIT.
Because some workers may be actively working on your HIT, MTurk holds money to pay these workers in case they complete the study and are approved. The money MTurk holds for these workers may cause your MTurk balance to dip below the amount needed to re-start the study. Usually, if you wait for the workers who are active in your study to finish—they can only take as long as you have set for the "max time allotted per assignment"—the funds associated with your account should re-calibrate and you will be able to restart your study.
If you continue to receive the "insufficient funds" error after waiting for the funds to re-calibrate, please contact our customer service.