What kind of audience are you targeting? Prospecting or Retargeting?
Audience Size: If your audience size is low or too small, you will be unable to spend. Increase the size of your audience by removing some targeting overlays.
Bid: If your bid is too low, you may have trouble spending. Increase your bid using either max bid or target bid, to try and reach more users.
Pixel Implementation: Confirm that your pixel is passing back item_id and that the item_id matches the ID attribute in your catalog. Learn more about pixel implementation here.
Audience Size: If your audience size is low or too small, you will be unable to spend. Increase the size of your audience by removing some targeting overlays and increasing the size of your product set.
Bid: If your bid is too low, you may have trouble spending. Increase your bid using either max bid or target bid, to try and reach more users.