[meindert]

I try to use setDefinedName to reference a worksheet-level value for a sheet with a space in the worksheet name:

String cellName="'APA Report'!NVEH01";
book.setDefinedName(cellName, "12.5434");
reportSheet.setFormula(2,4,cellName);

This code results in:
com.f1j.util.F1Exception: Invalid name.

The API documentation says;
When you create a defined name, the following rules apply:
■ A worksheet-level defined name begins with the worksheet name, followed
by an exclamation point. For example, any defined name on the Expenses
worksheet must begin with 'Expenses!'

How can I use a worksheet name with a space?

[RussW]

Try adding the defined name without the single-quote characters, e.g.:
String cellName="APA Report!NVEH01";

Quotes are needed when referring to the defined name within a formula.