How to add an alt tag to the logo?

are you using a svg graphic for your logo?
In that case the img tag is replaced by an inline svg file. The alt ( and title ) tag is not transfored to the inline svg – because this is not the way svgs are handled. Instead the most svg files do have title tag inside and f.e. a description:

<svg role="img" aria-label="[title + description]">
  <title>[title]</title>
  <desc>[long description]</desc>
  ...
</svg>

if you want to do it the correct way for wai-aria : just open your svg file before upload and add these two ( title is commonly present: change it to your needs ) add description of that svg file.

BUT: these attributes are not needed inside svg – see : https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/Core

• This reply was modified 1 year, 11 months ago by Guenni007.

On your media library – did you insert on attachment details any title or alt attribute?

By the way @ThiloKiefer – this is a nice tool – to inspect a whole page – and the free-version is quite sufficient. (Screaming Frog)

Yesterday i had a similar message with that tool – and it was my logo image – but i used it additionally on a different place ( in my case in icon circle as center logo image) unfortunately the alt attribute from the media library is not taken over here.
or f.e.: in your footer widget – have a look if you place that image elsewhere – because that tool does not show where you placed that image – only the path of that image.