Brake piece grinding against fork
@Adam Rice is almost there, diagram and suggestion included for what to do now...

First, you need a curved 18mm diameter washer between the brake and the fork. Your brake is a Shimano clone, here is how the original Shimano drive works out:



Note that there is no serrated washer between the curved washer and the brake.

Secondly, you need a 27mm long pivot nut on the back. If your existing nut is 18mm then that is a front nut too, but not suited for your fork. If the nut is 12.5mm or shorter then it is for the back brake.

What now?

Visit your local bike shop on a quieter day of the week and ask for help from the workshop with your predicament. Either get the bike booked in (and expect to pay their minimum workshop time booking slot) or buy the 18mm washer + 27mm nut and do it yourself.

All bike shops have a drawer full of such parts, they may charge ~50p per nut/bolt/washer which is not representative of the cost of the bit or their time to find it for you.

If you are buying something else, e.g. treat yourself to a new track pump, then they may be more helpful and do you the washer/nut out of good will.