libgit2sharp - Git - cannot push non-fastforwardable reference

I'm using libgit2sharp to push files to a repository once they have been generated.

using (var repo = new Repository(RepositoryPath))
{
    // Stage the file
    Commands.Stage(repo, "*");

    // Create the committer's signature and commit
    Signature author = new Signature("translator", "example.com", DateTime.Now);
    Signature committer = author;

    // Commit to the repository
    Commit commit = repo.Commit($"Resx files updated {DateTime.Now}", author, committer);

    Remote remote = repo.Network.Remotes["origin"];
    var options = new PushOptions
    {
        CredentialsProvider = (_url, _user, _cred) =>
           new UsernamePasswordCredentials
           {
               Username = _settings.GitUserName,
               Password = _settings.GitPassword
           }
     };
     repo.Network.Push(remote, @"refs/heads/master", options);
 } 

This has been working great for some time and pushing to repository without issue, however I am now getting the below error message for some files and the push does not occur for any files.

Error message (from server log):

Message: cannot push non-fastforwardable reference
StackTrace:  at LibGit2Sharp.Core.Ensure.HandleError(Int32 result)
   at LibGit2Sharp.Core.Proxy.git_remote_push(RemoteHandle remote, IEnumerable`1 refSpecs, GitPushOptions opts)
   at LibGit2Sharp.Network.Push(Remote remote, IEnumerable`1 pushRefSpecs, PushOptions pushOptions)
   at LibGit2Sharp.Network.Push(Remote remote, String pushRefSpec, PushOptions pushOptions)
   at Westwind.Globalization.Core.Utilities.DbResXConverter.SyncToGit(String resourceSet)

I have searched for error but cannot find anything that covers this? Also as this repository is sat on a Azure web server I assume I would have run any Git commands via libgit2sharp C# code as I would not have permissions to run command line Git commands.

Any help appreciated on this one.