Hello @AkshatRana, it looks like you are using java 9 or higher, the Appinventor source is only configured to run in java 8, running on a heigher java version would result in failure to compile the sources, See this answer here:
Hello @AkshatRana, it looks like you are using java 9 or higher, the Appinventor source is only configured to run in java 8, running on a heigher java version would result in failure to compile the sources, See this answer here: