"Helloworld" c program failed on my new ubuntu 20.04

C does not need openjdk. You need build-essential and make in this case. Simply type in the terminal sudo apt install build-essential make

I don't use eclipse so I don't know if you need to configure the makefile too.

Before proceeding you have to install needed build tools by

sudo apt-get install build-essential

For C/C++ development in Eclipse you have to install correct addon. Visit Help → Eclipse Marketplace and install Eclipse C/C++ CDT.

Then restart Eclipse to create project from File → New → Project, C/C++, C Project, Hello World ANSI C Project.
Finally build it by Ctrl+B, run with Ctrl+F11.