Hi @amarta and welcome the RStudio Community 
I'd like to give you a quick tip. Whenever you ask a question on this forum (and on any forum really), it is important that you provide all the necessary information for people to help you. In your case, you tried to estimate a linear regression. So, helpful information to provide would be:
- the relevant code and...
- your data or any data that would help run your code
This is usually referred to as a reproducible example. You may want to consult this great article to learn more about it: FAQ: How to do a minimal reproducible example ( reprex ) for beginners
Now, about your question, even though we have limited information, it seems that the source of your problem is: trade openness. R variable do not usually have spaces in them (even though it's possible). So what is exactly the name of that variable? It may be trade_openness or trade.openness or something like that!