The tooltip for the tool reads "Place no connection flag".
The only way to get around the double negative is therefore to suggest a remedy rather than to describe the error.
This is why I favour:
Pin not connected (use a "no connection" flag to suppress this error)
(The fact that there's currently a single-negative is just wrong, but doesn't much affect what the string should be.)
The tooltip for the tool reads "Place no connection flag".
The only way to get around the double negative is therefore to suggest a remedy rather than to describe the error.
This is why I favour:
Pin not connected (use a "no connection" flag to suppress this error)
(The fact that there's currently a single-negative is just wrong, but doesn't much affect what the string should be.)