Go to Settings>Compiler>Toolchain Executables. Click the "auto-detect" button for Compiler Installation Directory. It should come back with C:\Program Files (x86)\CodeBlocks\MinGW. Click the OK button and try again.
If it still doesn't work, it's probably because the MinGW path and executable names do not marry up with the Code::Blocks defaults in the 20.03 Windows binaries auto-install package once the install is completed. You can meticulously go through everything and fix it, but...
Rather than continue to struggle with this, download the previous release (17.12) instead and it should work properly.