Once you hit your plan's quota for properties, trail cameras, or photos, you will no longer be able to add the item that is maxed out until you take one of these steps.
1 - Delete Items
Deleted items do not count against you, so you can easily delete photos if you have reached your photo quota.
For example, if you delete 1,000 photos, you will add 1,000 photos to your plan. Just select the "Delete Photos" button, select the ones you want to delete (or press "Select All"), and the photos will be deleted.
2 - Upgrade Your Plan
Another option is to upgrade to a higher plan. To do this, select the gear icon at the top right like seen below.
Select "Change Plan."
Then select the plan that has more of what you need.