ERP system is huge and complicated. You must first know which parts of the ERP system you want to develop, so that you can know what technology you need and what knowledge you need to learn.

For simple ERP, the general architecture can be C/S or B/S mode, with 3 or N layers, including client/browser, server, database, terminal data and data reprocessing.

From these aspects alone, what you need to learn is:

Server-side development language: ASP, JSP or PHP, etc.

Client development languages: Java, C++, C#, VB, etc.

Browser-side development languages: HTML/CSS/Javascript and so on.

Database technology: mysql, Oracle, etc.

Auxiliary technologies: data communication format, network communication technology, distributed system technology, etc.

Data acquisition technology: embedded development, bar code, 2D code, RFID, etc.

Data analysis technology: data mining, OLAP, CBR, etc.

Enterprise financial management knowledge: financial accounting, financial statements, etc.

In a word, you need to master a lot of knowledge to be an ERP system. For example, SAP in Germany is the most successful ERP software supplier in the world, with over 16000 local employees and over 1000 engineers involved in ERP software development. Their ERP also includes human resource management, customer relationship management, risk management and so on.