Why does this error show up?

[AC Question]

Error:Execution failed for task ':ApplicationrocessDebugTestManifest'.
> com.android.manifmerger.ManifestMerger2$MergeFailureException: org.xml.sax.SAXParseException; lineNumber: 20; columnNumber: 6; The processing instruction target matching "[xX][mM][lL]" is not allowed.