What is 'Attempt Capture - Subscription Exists' log means?
It means the Nifty Direct Debit add-on found a Subscription ID attached to the particular client product in the invoice.
The Nifty Direct Debit add-on will not Attempt Capture when Subscription ID is mentioned for the client product. This is to update invoice status for subscriptions configured directly in GoCardless. When a Subscription ID is applied for a client product the add-on will log 'Attempt Capture - Subscription Exists' for information purpose.
In some cases, your previous add-on might have added the subscription ID automatically.
What if I leave it as is?
If you would like to leave the subscription IDs, then the Nifty Direct Debit add-on will update only the invoice status depending on the GoCardless subscription status. If you would like to use Attempt Capture for the product's subscription it wont work.
What should I do to use Attempt Capture?
You need to go into required client product in WHMCS and clear the value in Subscription ID and stop / delete relevant subscription in GoCardless.
IMPORTANT: You may have to send emails out to your customers beforehand as GoCardless will send emails out to your customer when you cancel the subscriptions.
