R install_github: Error: Failed to install 'unknown package'

I'm new to GitHub and I'm stumped. I can't get install_github to work.

I followed the helpful instructions here to set up my account: https://kbroman.org/github_tutorial/pages/init.html. I have an account and a repo where I store some R ggplot settings: https://github.com/tgraybam/TorontoPlot.

OK. Now I'd like to install the settings file from my repo into an R project, thus: devtools::install_github('tgraybam/TorontoPlot').

But executing the command returns an error:

Error: Failed to install 'unknown package' from GitHub: HTTP error
404. Not Found Did you spell the repo owner (`tgraybam`) and repo name (`TorontoPlot`) correctly? - If spelling is correct, check that you
have the required permissions to access the repo.

I have devtools installed. I am not spelling anything wrong. It is my own repo. I can clone it just fine:

git clone https://github.com/tgraybam/TorontoPlot.git

I can use install_github with different repos (this works fine: install_github("kbroman/broman").)

I can authenticate. ssh -T git@github.com returns

Hi tgraybam! You've successfully authenticated, but GitHub does not provide shell access.

Just like it is supposed to.

I've Googled up a storm and tried everything I can think of (multiple times in multiple orders).

I'm using a Mac. Here are my system and R details:

platform       x86_64-apple-darwin15.6.0   
arch           x86_64                      
os             darwin15.6.0                
system         x86_64, darwin15.6.0        
status                                     
major          3                           
minor          5.2                         
year           2018                        
month          12                          
day            20                          
svn rev        75870                       
language       R                           
version.string R version 3.5.2 (2018-12-20)
nickname       Eggshell Igloo         

Can anyone out there help? Huge thanks.