Why I'm not able to create new Projects and Tasks?

In the Projects section, there is an option to Disable a new Project and Task creation, meaning that the users won't be able to create any new projects or tasks and use only the existing ones. 

The setting can be edited only by the account's Admins or Owner.

To enable the option, go to Settings ➡ Account and make sure that the 'Disable Project creation' box is checked.

Instead of disabling this option Account-wide, you can restrict specific users from Creating New Projects or Tasks instead. In this instance, go to Settings ➡ Team Members ➡ Edit, and disable Project creation from there.