Hello I can't open Indesign 16.4 - when I try to open it comes up with this error message saying: "Your previous InDesign session has yet not terminated. Please try to launch the application after some time."
Sorry to hear about the trouble you are facing. In addition to @Willi Adelberger's suggestion, you may try opening Activity Monitor and check if there is a process open for InDesign. Then, try to force quit it and then relaunch the application. I hope it helps!