Why is there no option to install ubuntu alongside windows 10
You need unallocated space for that option to show.
If you do not have that you will need to shrink one of your partitions (to the size of the Ubuntu installation you want to use; at least a minimum of 30Gb).
Windows 10 that is preinstalled on a laptop is normally installed in UEFI mode. In the Boot tab of the BIOS settings uncheck the Legacy option making the USB flash drive bootable in UEFI mode.