When you add a chart page to a chart with a trading strategy already made for previous chart pages, the new chart pages’ trading strategy must be backtested. That is what is happening now.
If you want to abort this backtest for the new chart page you may press the Abort button at any time. If you do this, the trading strategy backtest results that you aborted will not be calculated; however, the trading strategy will show up on the chart page. If you want, you can manually backtest this trading strategy at a later time by Modifying Trading Strategies and selecting the Back button and then the Next button.
Topic of Interest:
What are Trading Strategies?