If you are seeing the 'Insufficient Funds' message in the app, the charger is rejecting your start request as it hasn't received confirmation your account is in credit.
To rectify this, firstly go to the 'Profile' page, at the top of the page underneath 'Account' you should see a 'Balance' value, if this is below £2.75, the charger will reject your account as having insufficient funds to pay for the charge cycle. You can add more credit by selecting 'Add credit' on the 'Profile' page, more information about topping up can be found here - Paying For Your Charge.
If your account is in credit over £2.75 and you are still receiving the the insufficient funds message, it is likely either your phone or the charger are experiencing connection issues and is unable to verify your balance. The quickest solution for this is to log out of the app and back in. This forces the server to refresh your account and resolves the issue.