Theme manager compiling bug

I want to set custom font sizes for h2 for example.
But I get the following error

image2338×1151 166 KB

(Bootstrap 5 latest beta, local)

Reproduce:

1. Go to theme manager → advanced
2. Select h2 font size
3. Change the value from $font-size-base * 2 to $font-size-base * 3
4. Compile with the rocket icon
5. Error occurs

PS in the screenshot I put it back to $font-size-base *2, which removed the line in theme.json, but still gave me the error.