I don't know what error code 18 is, but I would start with rebooting my computer. If that doesn't fix it I would try a different download method. If you are installing it from last year's program, try logging into your account and install it from there. If you are installing from your account, try installing from last year's program.
and ex marks the spot where those rocks and anvils hit me.
Make sure Win 10 is completely updated, and you dont have any updates sitting waiting to install...wouldnt hurt to turn off your anti virus for the download/install either, just in case its being a bit overzealous with its protection measures.