If your client has opened their proposal and can't see a signature block, there are two reasons why this might happen:
Maybe you forgot to add a signature block to the proposal. These aren't included automatically as not every proposal/document needs signing. You can read how to add one here.
It's possible that you didn't mark that particular recipient as 'Required to sign'. You can check this by clicking on the proposal from Documents > Outstanding and clicking 'Re-send document' - here you'll see a toggle next to each recipient. Make sure this is switched on and green, then click 'Save and Exit' in the top right to update everything.
When each of these issues has been resolved, you can either re-send the proposal to your client or simply ask them to reopen/refresh it to see the latest version.