Sometimes our merchants can't find the ad account they'd like to use in the list of eligible ad accounts because they haven't claimed their ad account yet. To claim an ad account, you can follow these steps -
You’ll need to have a Business Manager account. If you don’t have one, go to: business.facebook.com to create one.
Go to Business Manager Settings to claim the ad account
Go to Advert Accounts (sometimes called: Ad Account)
Claim your Ad Account
You will then be prompted to enter the Ad Account ID here:
You can find your Ad Account ID in the URL params when you are on the ad account. The account id will be the numbers after ?act=XXXXXXXXXXX in the URL.
You should now have your ad account assigned to your business manager!