I have been able to upload from other financial institutions. I checked with my representative and they can upload to windows on turbo tax. Another client is having the same issue on a Mac. This is the reply that I received from my representative.
reply from Northwestern Mutual
Thanks for reaching out on this and apologies to hear about the troubles so far. We have heard from one other client having difficulty with uploading their 1099 and that individual was also using a Mac.
We haven’t heard any issues from those using windows (and I tried my own this morning to confirm it was working and it was) so I do believe this is related to your laptop being a Mac.
We've reached out to the home office and they have confirmed there isn't any issue on their end, as they've tested the upload feature, and so believe it is a TurboTax/Mac issue.