Why I can't write to a tag?

Hello, I’m using a Tab Strip component to navigate between various windows, and I want to reflect the selected window in a label on other Window. I’m trying to pass this value (TabStrip.SelectedTab) to a memory tag, and then asign this tag to the label’s text.
I’m doing this by scripting, in the “Set Tag’s Value” tab, but when I run the program and try this, an error message pops up, and doesn’t let me do this. Why is this happening? Thanks.

What’s the text of the error message? Go to the ‘Details’ tab and copy the full trace of the error message, then paste it in here inside three literal ‘grave’ characters: `