Assume I want to add a check that the selling price or the price after a month of purchasing a share is 20% more than the buying price how do I express it in your expression language.
I am not sure if I understood your query. But if you want to close your trading position when a target percentage profit is reached then you can do that in Stockalyze. Please see attached image that shows the exact field in backtesting dialog where you need to supply the target value.
If there is anything else, please feel free to ask your question.