Bubble doesn’t support parentheses in dynamic expressions, so what you are building up is a mix of text and dynamic expressions, the parentheses being in the text part.
It looks like the Math.js plugin is giving you a small text field to work in, which makes it harder to navigate than a long text field, because once it sees the dynamic expression, it doesn’t let you click outside it to add text.
So the trick is to add the surrounding text first:
Then put the cursor inside the text, (between the parentheses), and choose dynamic text, click in the Bubble pieces …
(Current Page DiviSystem’s Copper * 0.1
Then you can edit the text on either side, putting in more text or dynamic expressions where you want them, so long as there is text on the edge you can keep adding more.
What Bubble will do is evaluate the dynamic expressions, concatenate the results with the text bits, and send the whole lot as a text to the plugin.