A few follow-up questions to help you troubleshoot the issue:
Are you able to build the package with devtools::build()?
What happens when you test the package with devtools::test()?
What file contains the line suppressMessages(require(testthat))? You can search the entire package in RStudio with Edit->Find in Files... (Ctrl-Shift-F). Can you share the contents of that file?
When I ran use_devtools() and copied the text over to .Rprofile, the text terminated without a carriage return. The solution was to simply add said carriage return to the end of the file.