Fixed---IDE Make *** *.elf error, but need to be explained

For weeks, I have been working on Intel® MAX® 10 FPGA Development Kit. Everything was Okay.

Till I add a Uart Rs232, Nios Eclipse IDE shows "Make *** *.elf error".

The reason is I connect everything for this UART in Qsys except IRQ because I don't need and also want to save some resource( actually I have plenty).

could anybody explain it?