The error you get gives you the remark that you did not set a alternate text for a picture. If the picture is not shown (for instance on a mobile with expensive connecting to internet you can set pictures off, this save data and time -> therefor money for the user)

In that case there wil be a text shown instead of the picture, that gives a clue about the picture that is missing.

This text you need to fill in with a text that describes the picture so the user has an idea whats going on.

With the goto page button you end up on the page where the text is not filled in. The nr behind the error, tells you where the object is in the page. (if you have a grid of 9x9 then 1,1 means top left object)

So doubel click the object, fill in the field with aletrnate text and youre done.

BR André