Problem with rss feed validation

Hi everyone ??
I’m quite new to wordpress, I know almost nothing of css and *absolutely* nothing about php.
I have troubles validating my weblog’s rss feed (both with wp-atom.php and wprss2.php): when I try to validate them through a validator, I get this error message:
“XML Parsing error: <unknown>:4:0: xml declaration not at start of external entity”
here are the urls to my files:
https://melyanna.altervista.org/wp-atom.php
https://melyanna.altervista.org/wp-rss2.php
Could you please help me fixing them?
Sorry if this topic has been already discussed: I tried browsing this forum before posting and found nothing.
Thank you in advance,
Kiara