Hi tobby,
Sorry for misreading your post, I took it for granted that you were trying to instal the driver. Sorry.
The lastest CUDA version I have installed so far is 10.0, which was done on Ubuntu 18.04.2 a couple days ago. I always followed the official Linux installation guide where the ‘root authority’ is a must because the toolkit is to be installed in /usr/local/ by default. As to your problem, I don’t know why you were not using sudo command but I still think the ‘root authority’ is a must according to your post.
Still, that’s my rookie view and hope you can find a solution after all. Good luck!