What’s the most efficient way to create a pull request?

What’s the most efficient way to create a pull request?

I’m currently making a code migration for NodeJs but I’m facing weird problem with git, I have the main branch where old code is located, the new code updated code is in another branch, when I try to make a Pull Request from my feature branch to my main branch, git says that there is no changes added, when more that 5 files and 300 lines of code were changed. I’m I doing something wrong?

Due to a poor branch strategy the main branch is far behind dev and the corresponding pull request contains over 500 commits.