Province isn't mandatory. Only the following invoice fields are.
First Name
Last Name
Address
City
Zipcode
Country
Believe this is because not every country provides provinces. You can however modify the invoice address XML file below and make it mandatory if you want, but I'm unsure if CBSubs will validate it and error if it's missing.
components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/xml/edit.front.invoice.xml
This is done by adjusting the first instance of the following.
FROM:
Code:
<param name="address_state" type="xml:state" blanktext="--- Click to select state ---" size="" default="" label="State" description="" />
TO:
Code:
<param name="address_state" type="xml:state" blanktext="--- Click to select state ---" size="" default="" label="State" description="" validate="required" />