Hi @Melissa_Rogans, welcome to the ODK forum! When you get a chance, please introduce yourself here.
This error is a little subtle. pyxform, the library that XLSForm Offline relies on for the conversion, uses the filename (not the title) when it creates the XML nodes and XML has certain restrictions on naming. Just to test this theory, try renaming your XLS file to just Test
and see if that works.
We have a fix for this in pyxform at https://github.com/XLSForm/pyxform/pull/152 and I'll see if I can convince the pyxform team to get this into the next version!