Hi YukariKudo_BE, welcome to the openmc community.
The coeffs that openmc asked were the surface coefficient being used for each surface type. So the xml file should look like this
<surface id="1" type="x-plane" coeffs="0.0" boundary="vacuum"/>
<surface id="2" type="x-plane" coeffs="1.0" boundary="vacuum"/>
Also, I recommend you to use the pythonAPI feature, i.e. basic functions as shown in the manual openmc – Basic Functionality — OpenMC Documentation
You could see some examples from the openmc notebooks git
GitHub - openmc-dev/openmc-notebooks at main