Ah okay you are on a free plan which has some restrictions
Have a look at: Deleting team member from free version - #2 by Emily_Roman
or Setting up an organization in Asana | Product guide • Asana Product Guide
And if you want to remove somebody from the workspace: Deleting team member from free version - #4 by Jenny_Brearley
If nothing helps I recommend reaching out to support as they have more insight into your account directly 