Olá Jefferson,
I was thinking of some bug on Telnet library, but checked the code and looked fine. Then I went to see your code and identified the problem.
You are having problems with spaces separation either on the definition of host and port, or in the Open Connection
call.
I have used RIDE and could fix the spacing and run the test case.
Here is the code, but with a different port:
*** Settings ***
Documentation Telnet Test Concept
...
... Test Pilot for using Robot Framwork to validate Telnet server
Library Telnet
*** Variables ***
${host} localhost
${port} 80
*** Test Cases ***
Connect to Server using Telnet
Open Connection ${host} port=${port}
Close All Connections