Select a signal line in your model, right click, select Properties, type in a name "SignalA", and check the check-box "Signal name must resolve to signal object".
Then it requires you to create a Simulink.Signal object in the workspace, such as SignalA=Simulink.Signal and then specify some property values. If the signal object does not exist, or variable "SignalA" exist but it is not a signal object, then the error in your post will appear.
It sounds like you are not aware of this technique, then most likely you don't need it. So remove the signal line name inside the library block.
This technique should not be used inside a library block anyway. For the same signal name (so is the signal object name), it only allows one instance in the whole model. So if you implement this inside the library block and it is used multiple times, then another error will occur.
