Your keto jello could end up runny for several reasons:
Poorly dissolved gelatin: Make sure you whisk your gelatin completely so it reaches full strength.
Too much water: Measure your liquid carefully (you need 3 cups total at the end of step 4), otherwise the jello will get too soft.
Not refrigerated long enough: Gelatin needs time to chill and firm up. Make sure you give it at least 4-6 hours, or ideally let it set overnight.