error on line 3 at column 6: XML declaration allowed only at the start of the do

I have the same problem but

error on line 1 at column 6: XML declaration allowed only at the start of the document

I removed all the white space from my theme functions file and it seemed to have resolved the issue.

wich all white space ?

Many of the forums suggested analogous solutions similar to:

https://www.arnebrachhold.de/projects/wordpress-plugins/google-xml-sitemaps-generator/help/
https://premium.wpmudev.org/forums/topic/error-in-generated-sitemapxml-google-webmaster-tool-and-browser-check
https://www.piotrkrzyzek.com/wordpress-remove-blank-line-from-rss-feed-wordpress-error-on-line-2-at-column-6-xml-declaration-allowed-only-at-the-start-of-the-document/
https://www.youtube.com/watch?v=PGathYQHHzk

So I removed all the line-breaks after the last php bracket in custom-functions.php and it seems to have resolved the issue, but still not sure.

but in wich file should I make this?

in functions.php is all okay

Did you remove white space after the last php bracket?

Do you use a child theme? If so, did you look at custom functions file?

I use my own theme. And yes I remove white space after the last php bracket, in functions.php

Did you look at the other solutions? Also, what do you have your file permissions set as?

wich solutions are you minds?

and wich file shoud I permission look of?

look at links above. With the exception of a few files, I set almost all the files to chown user:apache -R /wordpress and chmod 755 -R /wordpress. But this might be a red-herring.

Somewhere there is still a white space left. Please see here for a possible workaround if you can’t find it.

https://www.arnebrachhold.de/projects/wordpress-plugins/google-xml-sitemaps-generator/help/#P221_gwtinvxml

Using a child theme?

same issue i fetch. for this issue it is clear that somewhere blank space exist. to solve i suggest 2 solutions
1. Find out white space / line break and remove it
2. WordPress leading whitespace fix @ https://wejn.org/stuff/wejnswpwhitespacefix.php.html