Ext JS is dual licensed GNU GPL v3. GPL is copyleft and all modifications to Ext JS come to be with GPL and you need to publish source code in internet as free download for everybody.

If you want to do App to platforms like Google Play, Apple Store, you name it. Then you need to purchase commercial license for Ext JS.

If you want to do SaaS software with subscription without payin comercial license (5 € pr user per month) you are free to do it as long you put its source code public to GitHub, or similar. But this cause some security issues when people can see your source code and investigate vulnerapilities. Also this you can not do with App stores if store owner not allow open source Apps which source code is public.