1. What is this error and how to fix it?

This weird error, similar to the photo below, appeared on my Truenas Scale install after importing some old Truenas Core pools.

To fix this error, simply navigate to System Settings on the Truenas Dashboard:

And then click on Shell:

And then enter whoami

If the output is root as shown below, keep going.

If not, type sudo -i

Next enter zpool status -v

This should output the status of every drive pool you have got on your system.

WARNING: If you execute the following command on Truenas Scale, the pool CANNOT be imported back into Truenas CORE. It will fail, and all your data may be lost. MAKE UP YOUR MIND

To upgrade the ZFS pool, enter zpool upgrade followed by the name of the pool stated here:

So the command here, should look like: zpool upgrade mystoragepool

Do this for every pool and then reboot

After Truenas boots, this error should no longer be there!!