If you have tried to install DirectAdmin on Ubuntu and received an error related to GNU C Library such as:

/usr/local/directadmin/directadmin: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /usr/local/directadmin/directadmin)

This implies that you have chosen the wrong OS on the ordered license. Ubuntu isn’t yet specifically supported therefore choosing Ubuntu or Debian on the drop-list may not be helpful. Configure rather your license for Linux 64bits.

P.S.: If you encounter a different issue, search directly on DirectAdmin Forums, don’t rely on global search engines.