I am assuming you have both the app and the Echo registered to the same account. Even if you have renamed it the command "Alexa turn on first plug" should work. If you have renamed it then "Alexa turn on light" (or whatever) should work as well.
Have a look at the web controls Alexa smart home to see if it is listed and if so use "discover" if it is not there you may be on a different account.
I solved my issue whereby the echo voice command would not recognise the smart plug after deregistering from my account and reregistering to my wife’s. it would however still turn on and off via the app. To solve this I simply said ‘echo, change account’ and she confirmed the switch to my wife’s. I don’t know why this step was necessary as it was already registered in my wife’s account and the app indicated it was too. I hope this helps someone else.