I have OnePlus 6 with latest Open Beta. It is rooted. I have done everything but I still can't install any app as a system app.
I have tried Link2SD and Titanium Backup. I have tried to copy/paste the apk using TWRP file manager and ES file Explorer. I have tried to move the apk using adb shell. Everything is giving the same error that /system has no space left.
Link2SD storage shows
Settings storage shows
Why it is giving me memory error when system has space?
... /system has no space left.
It indicates that your /system partition is mounted R/W but just to make sure, if dm-verity is enabled, /system is always R/O. For details see How to disable dm-verity on Android?.
Next, both of your screenshots show that your /system partition is full. Even if you haven't written anything to it, some ROMs may have /system already completely filled. After all it isn't for user's usage.
The space shown as "System" in Settings isn't actually the /system partition. In fact it can't be of 11GB, that's too much. Android is counting a large data from /data partition also as "System", because it's not what a "user" directly puts there. User can access only shared/external storage (/sdcard).
You can check actually space usage of /system partition with command df -h /system. Execute du -h /system | sort -h to get a directory-wise detail of what's taking up space.
For more details, see How disk space is used on Android device?