When you are trying to install elementary OS . The system gives you the option between installing automatically with Windows and advanced options where you can do the partitioning.
I suppose that you chose the second option, there you can find at the bottom and before you start installation, an option asking you where you want to install the boot loader, but there you have to choose the disk and partition where you want to install it, try changing
/dev/sda1 (obviously only if
/dev/sda1 is one of the Linux partition, not Windows one)
As an example, years ago i was installing Kubuntu and my computer never boot, i put my booteable sd-flash for reinstall but i didn't change the priority boot, and computer finally start, then i realize that i had installed the "start sector" in the sd-flash that was at the instalation " sda ".