So the library that pihole-FTL depends on is "/lib64/ld-linux-x86-64.so.2".
Is it installed ?
dpkg -S ld-linux-x86-64.so.2
And is it at the expected location ?
whereis ld-linux-x86-64.so.2
stat /lib64/ld-linux-x86-64.so.2
That library is part of the "libc6" package.
If not installed, you can do so with below one:
sudo apt-get install libc6
And check again if that library is at expected location now with the "whereis" and "stat" commands.