If you are using prepay Facebook ad account to run Facebook adverts, the billing section of your account will display your prepaid balance which is simply the amount of money available in your account to pay for your Facebook ads.
The Naira Facebook ad account I use for Facebook advertising is a prepay Facebook ad account. I can only run ads with it when I have money in the prepaid balance unlike my dollar Facebook ad account which allows me to run ads and pay later at the end of the month or when my ad spent reaches the limit set for me by Facebook.
If you are using such prepay Facebook ad account, you are required to add money to the balance manually. Hence, you should always ensure you always have enough balance there that will be enough to pay for your Facebook ad campaigns.
Whenever you want to check your Facebook ad prepaid balance, sign in to your Facebook Ads manager and click here or just navigate to the billing section.
On the Facebook ads billing page, you will see your balance displayed like this :