\begin{center}
\begin{forest}
for tree={circle, draw,
            minimum size=2em,
            inner sep=0pt,
            s sep=2mm,
            l sep=1mm}   
	[$\lnot$, name=not_one
		[$\lor$, name=or
			[$a$, name=a]
			[$\lnot$, name=not_two
                [$b$, name=b
           			[$\lnot$, name=not_three]
                ]
			]
		]		
	]
\end{forest}
\end{center}

One leaf is labeled with a logical operator, which is not allowed.
Thus this is not a well-formed formula.