We have seen this on a number of occasions but can't pinpoint the exact steps to reproduce. Sometimes NinjaTrader when using the free demo key will ask for a license key. This often confuses people but we can help you get back up and running.
What License Key Should You Use?
Do you have NinjaTrader 8 installed and it's asking you for a license key (for the demo version)? We used to post a key but NinjaTrader has told us its best to get a unique key from their support which will still let you use the demo version for as long as you'd like to use it.
This is NOT a lifetime license key, but is for the demo version which you can tell since the key starts with @SIM (for simulation).
Contact NinjaTrader support and ask them for a new @sim key - they will provide this without issue.
If The Key Doesn’t Work?
Should the key not work I'd recommend talking to NinjaTrader about this because you might have a live license key and need to use that instead. This would have required you to purchase a NinjaTrader 8 license key.