An app build does not appear in TestFlight

Check email linked with apple account, you might have received email listing with issues apple found on your build which most of the times related to .plist file for any missing permission description. After removing issues do change the build Number as you cannot upload with the same build number as previous one.

I could fix the problem by adding this key "App Uses Non-Exempt Encryption" in Info.plist

App Uses Non-Exempt Encryption = NO

Besides the reason mentioned by Ali there can be one more thing. Your application might be on processing for the time. Just go to:

Your application>TestFlight>Activity

Check if application with uploaded build version is there and status is in processing. It may take up to and hour or more before it come to your testing tab.