On submitting proposals especially to those that are fixed contract I always get the message to Purpose Different Terms.
I am trying to find out because I the message keeps coming back even if I change the bid to a smaller amount.
You'll see "Propose Different Terms" button on each of your proposals. It's there in case you want to change your bid. You don't have to use it. Note though that Upwork has a minimum hourly rate of $3/hour and minimum budged of $5.