Add element to posts by specific author

Can you provide a link to the Author Page ?

The author page and the archive page seem to be in the same state.

Please check it out.

Try changing:

if ( 553 === $element_id && '2' == $author_id ) {

to:

if ( 553 === $element_id && '2' == $author_id && is_single() ) {

This will set the condition to only the Single Post.

It worked, thank you!

Glad we could be of help