wordpress Error on line 2: The processing instruction target matching "[xX][mM][lL]" is not allowed.

this error really showed me how outdated the links on google search are . All the solutions were not working

Error on line 2: The processing instruction target matching "[xX][mM][lL]" is not allowed.

If still, your problem exists furthermore you probably have to check above 3 possibilities in wp-config.php, wp-rss2.php, wp-atom.php files. https://speedysense.com/wordpress-blank-line-before-xml-declaration-rss-feed-does-not-validate/

These files did not even exist in the latest version of wordpress. Finally it was the theme which caused the error . Changing the theme to the default theme fixed the issue