Hi! Are you, by any chance, using JDK9 or JDK10? Currently only JDK8 is supported, so if you happen to have JDK9 or 10, I’d suggest uninstalling it and installing JDK8 instead.
It’s a known issue that affects android development in general and we’re working to solve it. ( Java 9 (JDK 9) support by Unity Android )