Plum tree is covered with sap
Bark on young plum tree is covered with resin-like sap.
It might be bacterial wetwood or peach borers or canker, depending on how the sap looks. These articles will describe the nature of the running sap for each problem and will suggest solutions: https://www.gardeningknowhow.com/ornamental/trees/tgen/wetwood-bleeding-trees.htm
Additionally, it could be sap-sucking insects, like aphids, which leave behind sticky honeydew. Here is more information: https://www.gardeningknowhow.com/garden-how-to/info/what-is-honeydew.htm