I found a similar issue on Google and it seems that you need to edit the bitcoind.service because it is configured as user: bitcoin and group: bitcoin by default you need to replace it with root if you were currently running it on root user.

And also I'm pointing out this setting below and you set it to true which is why the error denied access popup while currently running it on the root user change it to false and try to run it again.
"# Deny access to /home, /root and /run/user
ProtectHome=true"

Here's the source below that I found and hope you can get some idea how to set up bitcoind.service properly.

- https://bitcoin.stackexchange.com/questions/118725/bitcoind-service-cant-start-on-mounted-drive