I keep 2 different Businesses in Manager. The problem right now is that only one of those is allowing me the option of modifying the Sale Invoice Template. I really urge to modify the other one. How can someone help me?
The ability to create sales invoice templates was removed a while back from the program due to technical issues. I can’t say what the developer has planned long term regarding customisation of templates but something is in the pipeline. Using the custom templates is not recommended as new functionality or changes to the program are not being applied to the custom templates due to support for custom templates being dropped.
You can still create custom templates under Settings at View Templates. What was removed for newly created businesses under Sales Invoice Template is the ability to (1) rename the sales invoice, (2) add a prefix to invoice numbers, and (3) standardize terms and payment advice. But you can still enter terms in the Notes field and make default text. Existing businesses that already had settings under Sales Invoice Template can still use them.
I stand corrected 
The custom templates are still available. However, I would still advise not using them as they are becoming deprecated in favour of a new way of customising templates in order to eliminate technical issues for the developer.