Yes, even I faced the same problem. I kept on trying daily. Finally after few days I was able to register and that error was gone. So, try daily.

done everything.. yet it says invalid

COAP 2018 registration will start from May 16,2018. For more details check the link below. 

https://www.google.co.in/amp/s/engineering.careers360.com/articles/gate-coap%3famp