You can find the instructions on how to install and configure the software required for the Fast Incrementals feature starting on page 36 of the User Guide:

https://support.emc.com/docu91839_Avamar-18.2-Backup-Clients-User-Guide.pdf?language=en_US

I wasn't sure what version of Avamar you're using so this is a link to the documentation for the latest release. Other versions will be similar. You can find the User Guide for other Avamar versions on Online Support.

I already followed this documentation. I think I have to prepare the mount points for linux fast incrementals but the documentation does not show how to do. I use Avamar v7.5.1-101

It sounds like you may not have have downloaded the "AvamarFastIncrementals" RPM from the Avamar server and installed it on this client system. On RHEL systems, this package places lfi-config or lfs-config into the /sbin directory. One other possibility I can think of is that /sbin may not be in your PATH, although this would be very unusual.

Edit: Unfix typo

Great! I found the lfi-cong in /sbin :)

I tried to prepare the mount points with 

[root@centos6 ~]# /sbin/lfi-config -c /dev/mapper/vg_centos6-lv_root
Error:Configuration library initialization failed

I tried also:

[root@centos6 ~]# /sbin/lfi-config -c /
Error:Configuration library initialization failed

[root@centos6 ~]# cat /etc/mtab
/dev/mapper/vg_centos6-lv_root / ext4 rw 0 0
proc /proc proc rw 0 0
sysfs /sys sysfs rw 0 0
devpts /dev/pts devpts rw,gid=5,mode=620 0 0
tmpfs /dev/shm tmpfs rw,rootcontext="system_u:object_r:tmpfs_t:s0" 0 0
/dev/sda1 /boot ext4 rw 0 0
none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0

But I don't know this kind of error :(

Sorry, my typo. The binary is called lfs-config, not lfi-config. Check if the Avamar Fast Incrementals package is installed, please:

rpm -qa | grep AvamarFast

If it is not installed, you will need to install it as described in the guide.

The package is installed but I don't have "lfs-config"

[root@centos6 logs]# rpm -qa | grep AvamarFast
AvamarFastIncrementals-linux-7.5.101-101.x86_64

[root@centos6 logs]# find / -name lfs-config
[root@centos6 logs]#

Okay, so I double-checked the contents of the 7.5.1-101 RPM package for RHEL6 and the binary is called /sbin/lfi-config. I apologize for the confusion.

When I run this binary on a local test system -- even without installing the RPM package -- it works without issue. I think there may be something happening at an OS level on your client system that is preventing the binary from properly loading shared libraries.

Please try running the following and see if it reports missing dependencies:

ldd /sbin/lfi-config

Here the output. On the other hand it is a test server that I use before working on the production. Do not spend too much time :) I will try the installation on the production server. I have to move on.

[root@centos6 ~]# ldd /sbin/lfi-config
linux-vdso.so.1 => (0x00007fff769a0000)
libext2fs.so.2 => /lib64/libext2fs.so.2 (0x00007f015aba9000)
libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f015a98c000)
libdl.so.2 => /lib64/libdl.so.2 (0x00007f015a787000)
libc.so.6 => /lib64/libc.so.6 (0x00007f015a3f3000)
/lib64/ld-linux-x86-64.so.2 (0x0000560876820000)
libcom_err.so.2 => /lib64/libcom_err.so.2 (0x00007f015a1ef000)