Since there have recently been multiple technical issues with lithium ion batteries in many products, US-wide inspections have been ordered. As a result, the batteries built into the PAPELL experiment must undergo additional inspections. As the required tests are time consuming, they will not be finished before the Cygnus supply ship's last loading date. Kirk Shiremann, NASA Manager ISS Programm, made the decision that an only partially inspected experiment is too much of a risk for crew safety, which means that the experiment cannot yet be launched.
The next opportunity for launch is the SpX-15 supply mission being launched from Cape
Canaveral on June 28th.