In Terminal, if you can run the following command:
diskutil info /dev/disk4 | grep Read-Only
It should come back with two lines. Look at the Read-Only Media line - it will say "Yes" or "No".
Repeat the command, substitute disk4 with disk3 and then disk3s2.
These commands should all return "Yes" or all "No". If it doesn't, that would be unusual. On a properly functioning SSD (with corrupt data or not), it should be "No". If a SSD has gone into read-only mode, it will read "Yes". If the SSD has gone into read-only mode, it's because it detected some fault and to prevent further (if any) data corruption, it went into read-only mode and there's a good possibility that whatever fault it was is what borked the ability to properly mount the disk. If it isn't in read-only mode, then it's more likely the OS or install program screwed up.
The -69808 error in the same context as yours doesn't show up much in web searches and I also saw one article where it showed up in a HFS+ disk.
You may want to make an image of the SSD - if the SSD is not in read-only mode, as you try to fix it, you may be further corrupting the data. Or if the SSD is in read-only mode, it may be that transferring the data, byte-by-byte to a new working SSD will fix the issue (although I wouldn't get too hopeful of that). The problem is that with a 1TB drive, working with an image is going to be cumbersome. Also keep in mind that writing a full image to a new SSD may take a substantial part of it's life - for example on a Samsung 1TB 850 Evo, if it does write every byte to the disk (don't know how it would handle 0-value bytes), you're using 1/150 of the SSD's life (newer Samsung and Crucial models have substantially higher endurance rating, but this example is just to give an idea of what this entails). If you want more information on imaging the disk, put a post to that effect.