Some possible reasons for errors are listed below . Pick up 4 most important ones and explain why they are the most important ones. State whether they are systematic or random and explain. Temperature expansion of ruler and thread. Wooden ruler is not enough precise - should be steel one.
Vibrations are in x and y direction.
Position of node is not clear. Weighing error - scale is not precise enough. Linear density is non uniform Tension is not constant. Frequency of vibration is not constant. Thread is not horizontal.