Can this ratatouille be saved?
What you're tasting is likely the eggplant -- eggplants with more seeds can have a distinct metallic taste, and can definitely ruin a dish.  There's probably nothing that can be done for this batch, but for next time:

Look at the bottom of the eggplant when you're picking it out -- if it has a small round indentation, then it's a "male" eggplant and will have less seeds;  if it's a larger horizontal indentation, then it's a "female", and will have a higher seed content (and ruin your Ratatouille).  Once you look at a couple, you'll see the difference -- they're distinctly different and easy to pick out once you know what you're looking for.  Google "male vs female eggplants" and you'll find plenty of pictures of this.

One note: the "male/female" nomenclature isn't exactly correct:  What they call a "male" is an eggplant that was less pollenated and therefore generated less seeds, though the eggplants don't technically have any gender -- but, this is the common terminology used to describe the difference.