You can enter a maximum of of 65,000 placement exclusions per account, and so using this full allotment will prevent Squared.io from being able to add any further placement exclusions.
Manager accounts can create up to 3 placement exclusion lists with a maximum of 250,000 exclusions per list which can then be applied to individual accounts, and so moving negative ad placements to here will free up account level space that can be managed by Squared.io.
For full instructions on how to set up Manager level ad placement negative lists, see this help article from Google, and to get a list of all current account level placements and how they were added (i.e. manually or automatically in Squared.io or manually in Google Ads), please used the download on the reports placement page: