The 'Sign-in with Steam' button will be visible if you're launching Robocraft from within Steam and Steam library is open. When logged into Steam click the Steam menu and select "Check Steam For Client Updates" this will force Steam to check for updates and download them if necessary.
Also, your Steam will needs to be set as ‘Online’ as well. This can sometimes cause errors with Robocraft not seeing the Steam client.
Please also follow these easy steps to check the sign-in with Steam button is visible.:
-Exit the Steam Client
-Ensure both Robocraft and the Steam Client are removed from the taskbar
-Load the Steam Client and login
-Go to the Library tab and click on Robocraft in your games list
The login button should now be there. If not, please restart your Steam Client.