UPDATE tbl_home SET tbl_home.IsActive = False
WHERE (((tbl_home.ProjectID)=[Forms]![frm_ProjectDetails]![txt_projectID]));
True and False are NOT strings. They are boolean values 1 and 0 although, Access uses -1 and 0.
Also, we have no idea what the context is or why you are trying to update a second table from I presume a form. Logically, this is probably wrong.
We also have no idea how/where you are running the query so the solution I suggested may not help at all.