Sometimes 'Jeg orker ikke' means 'I don't have the energy.' It can mean a lot of things.
If you say: 'Jeg orker å løfte 50 kilo.' you mean to say that you can do it, you have the strength to do it. If you say: 'Jeg orker ikke å spise mer.' you mean to say that you cannot eat anymore. To have the energy to do something is 'å orke å gjøre noe'. Children say 'Jeg orker ikke' to everyhting they are asked to do.
Mother: 'Gjør lekser.' (Do your homework.) Child: 'Jeg orker ikke.'
Orker ikke is something like cba in english. https://www.urbandictionary.com/define.php?term=cba
There is some helpful information here: https://forum.duolingo.com/comment/16850825/The-word-Orker