I think the problem is that your link is arborescence and that you use /Dinstead /J, which can handle network locations.
"u:\mobile apps\repos" contains 2 folders (mobile applications and repositories)
It seems that mklinkit cannot create 2 folders. Therefore, if you try like this:
mklink /J "U:\Mobile Apps" C:\Users\LeiceJ\Source\Repos he will work.
You can also manually create a folder called "mobile applications" and then it will work.
Another point: it seems that you turned the target over the link (but not sure). If you want to specify a disk card U:as the target during input "C:\Users\LeiceJ\Source\Repos", you need to invert two parameters in your command.
TL; DR: mklink /J , .