With Xcode 11 as command line tools, to validate or upload an ipa, replace altool with xcrun altool:
xcrun altool --validate-app --file "$IPA_PATH" --username "$APP_STORE_USERNAME" --password @keychain:"Application Loader: $APP_STORE_USERNAME"
xcrun altool --upload-app --file "$IPA_PATH" --username "$APP_STORE_USERNAME" --password @keychain:"Application Loader: $APP_STORE_USERNAME"
Get more help with xcrun altool --help.