This only affects people who are using a computer with a legacy BIOS that already has 4 primary partitions in use.
To fix the issue remove one of the 4 primary partitions.
Important: If you decide to remove a data partition make sure you have backed up the data first. If you decide to remove a recovery partition make sure you have created other recovery media
After deleting one of the 4 partitions you should be left with 3 primary partitions and an area of unallocated disk space.
When you run the Ubuntu installer you should now see the option to install alongside Windows 10.
If you do not get the option to install alongside Windows 10, choose the something else option as the installation type and create 2 extended partitions in the area of free space, the first taking up most of the disk space and mounted to root (/) and the second taking up around 8 gigabytes for swap space. The amount of swap space can be reduced or increased depending on the age of your machine and amount of memory available.