Unable to modify file permissions.xml, get error Read-only file system I have a rooted Galaxy S5 SM-G900V, lineage_klte, ARMv7 Processor rev 1 (v71), Kernel: 3.4.113-lineageos #1 SMP, Oreo 8.1 that a friend asked me to modify.

I need to modify the platform.xml file to allow the system to write data to the external sd card. I need the ability to modify this file. The current permissions are:
rw-r--r-- 1 root root, however I am unable to save any modifications.  

I am unable to modify the file permissions and would like to know how to do so. Note, I am looking how to change the file permissions and not how to or what syntax to use in the xml file, or if it will allow me to have the system recognize the external sd card for saving data. I am perplexed on why I am unable to modify this file.

&gt; adb devices  &lt;-- verify I have the device
&gt; adb shell    &lt;-- enter shell
&gt; su           &lt;-- 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
 &gt; 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
 &gt;&gt; chmod: chmod 'platform.xml' to 100755: Read-only file system


So, how do I change the file attributes for platform.xml allowing it to be modified? I thought that there was a hidden immutable attribute on the file but there is not(at least that I can determine).

Related question, is there any other website to post this question? Any help is most appreciated.
I was only able to remount rw after 
- removing addonsu (by sideloading in TWRP "su removal" from https://download.lineageos.org/extras)
- installing Magisk (easy, sideloading current release from https://github.com/topjohnwu/Magisk/releases)

I was under the impression that addonsu does not fully grants permissions -- after all I found myself heading down this road when attempting to configure Titanium Backup.

By the way, TWRP has a commandline guide at https://twrp.me/faq/openrecoveryscript.html , all the above is as easy as adb reboot recovery, then adb shell twrp sideload which makes TWRP waiting for a file, so just serve it by e.g. adb shell twrp sideload ~/Downloads/Magisk-v19.3.zip