Scam. Flag and report.
Almost certain the money will be to purchase from their "approved vendor", which is likely owned by them, and they're wanting to use you to launder the money.
(Stealing Preston's response from another thread...)
Scammers tend to prey on virtual assistants and people who are new to the platform. You're both, which is why they tend to target you. But you'll soon learn to sort the wheat from the chaff, and once you have more experience, they won't bother, because they won't see you as gullible enough.
Just keep your wits about you, and if you have any concerns, the community is a great place to ask. And I'm sure Petra will be along soon with her handy flowchart. It's a great tool to help figure out what's what.
Unfortunately, newcomers in your category are invariably targeted by scammers. You can be almost sure that any invitation you get to begin with, will be suspect. The best thing to do is to decline those invitations and to flag them as inappropriate so that Upwork can remove the job concerned. But you should also know that these scammers are legion on the Internet not just on Upwork. It is like fleas, you can spray in the appropriate areas, but you cannot completely eradicate them.
So the answer is to continue to send proposals to the job offers you know you can do and hope for a positive response, which eventually you will get, but it does take time and determination. Just be careful when applying, to avoid offers that are too good to be true. You are not on this site for a safe-safe employment package, and if this is offered don't waste a connect on it.
If you haven't already, have a look at this thread put together by Upwork and is a must-read: https://community.upwork.com/t5/New-to-Upwork/Getting-Started-on-Upwork/m-p/264214#M2460