You will have to do the following on your PC
On branch dev
$ git pull --no-rebase origin master – This will create a merge commit and you will have to resolve the conflicts in the files which are changed both on dev and master. git status will show the list of files with conflicting changes.
After resolving conflicts, commit all the changes and push your branch. After that you should be able to complete the PR.
FYI: –no-rebase makes sure that a merge is done even if the pull behavior is overwritten to default to rebase.
Help link for more details