Your query contains columns which could be present with the same name in more than one table you are referencing, hence the not unique error. It's best if you make the references explicit and/or use table aliases when joining.
Try
SELECT pa.ProjectID, p.Project_Title, a.Account_ID, a.Username, a.Access_Type, c.First_Name, c.Last_Name
FROM Project_Assigned pa
INNER JOIN Account a
ON pa.AccountID = a.Account_ID
INNER JOIN Project p
ON pa.ProjectID = p.Project_ID
INNER JOIN Clients c
ON a.Account_ID = c.Account_ID
WHERE a.Access_Type = 'Client';