As documented:

Full path to upload the file to. When uploading multiple files, the filename in the path should be replaced with operation mask or omitted (path ends with slash).

https://winscp.net/eng/docs/library_session_putfiles

I'm moving from a legacy batch file to a powershell script and ran into an issue when trying to execute $transferResult = $session.PutFiles($localPath, $remotePath), I kept getting access is denied. I triple checked my $sessionOptions and everything was correct. After hours of debugging turns out that I was missing a / in the $remotePath variable...

So this:

$remotePath = "/destinationFolder",

Needed to be this:

$remotePath = "/destinationFolder/",

Hope somebody finds this useful in the future.