If you want to manage an ad account, there's 3 ways to add the ad account to your Business Manager.
If you have already added a personal ad account to Business Manager. You can only add one ad account from your personal Facebook account into Business Manager. Instead, you can create a new ad account in Business Manager or you can request access to another ad account.
You can create up to 5 new ad accounts in your Facebook Business Manager.
To add a Facebook ad account to business manager
Go to Business Settings.
Click Accounts and click Ad Accounts.
Click + Add.
Choose one of the 3 options:
Add Ad Account: Adding an ad account moves it permanently into Business Manager. You must be both the owner of the ad account and an admin in Business Manager to add the ad account.
Request Access to an Ad Account: If you request access to an ad account in Business Manager, the admin of that Business Manager can grant you permission to work on it.
Create a new ad account: If you create a new ad account in Business Manager, it'll permanently belong to that Business Manager.
If you choose to request access or add an ad account, enter the ad account ID.