Help - Lost Mac Partition

[humbuck7]

Hoping someone here can guide me - I'm not very well-versed in these fixes but would love to see if this can be fixed.

I have a 2012 macbook pro, upgraded RAM & 500gb SSD a few years back. Boot disk is OSX 10.9.5
I created a partition 3 years ago so I could run a newer OS (macOS Catalina (version 10.15) required for some apps, worked great until yesterday

Now, I cannot boot into the new OS at startup. Running Disk Utility shows the partition name has been changed to "disk0s4". I cannot mount, unmount, or do anything within Disk Utility with this partition...

After some googling I discovered testdisk, and got it running, but as soon as I select a drive I get this message:

Write access for this media is not available.
TestDisk won't be able to modify it.

- No partition from this disk must be mounted:
Open the Disk Utility (In Finder -> Application -> Utility folder)
and press Unmount button for each volume from this disk
- This media may be physically write-protected, check the jumpers.

Can someone point the way to get write access fixed, and maybe walk me through the rest of the procedure so I can maybe get some data back?

Maybe worth mentioning, I believe I was able to copy something from the partition in question, it is saved as image.dd - is there anything I can do with this for recovery?

Thank you very much

[recuperation]

Testdisk gave you some hints that are worth following.
I would like you to follow this advice and give me a feedback.

Thank you.