adb – file permissions.xml can not be changed. Error in read-only file system

I have a rooted Galaxy S5 SM-G900V, lineage_klte, ARMv7 processor, version 1 (v71), kernel: 3.4.113-lineageos # 1 SMP, Oreo 8.1, which was asked by a friend to change.

I need to change the platform.xml file so that the system can write data to the external SD card. I need the ability to change this file. The current permissions are:
rw-r – r– 1 root root, but I can not save any changes.

I can not change the file permissions and I want to know how this happens. Note, I'm looking at how to change file permissions, not how and in what syntax the XML file should be used, or whether I can recognize the external SD card for storing data. I am amazed why I can not change this file.

> ADB devices  <-- verify I have the device
> ADB Shell    <-- enter shell
> su           <-- super user
# ls -l
drwxr-xr-x  17 root   root      4096 2019-03-20 10:26     system
# cd system
#ls -l
drwxr-xr-x 20 root root     4096 2019-03-20 10:25 etc
# cd etc
 # ls -l
 > drwxr-xr-x 2 root root 560 2013-12-31 21:23 permissions
# CD permissions
# ls -l
-rw-r - r-- 1 root root 9116 2008-12-31 18:00 platform.xml
# chmod 755 platform.xml
>> chmod: chmod & # 39; platform.xml & # 39; to 100755: Read-only file system

So how can I change the file attributes for platform.xml so that it can be changed? I thought there was a hidden invariant attribute in the file, but it does not exist (at least I can tell).

Related question, is there any other website to post this question? Any help is greatly appreciated.