That is a restriction on the UEFI shell open core uses. You may want to try older shells like the refit shell. Most of the EFI tools requiring text mode do not work in the opencore shell.
This is missing a bit of nuance and anything requiring basic console text will work in OpenShell.
OpenCore does not provide the environment needed for a shell to use text mode display (everything is graphical, including the text bootpicker) but once you boot into OpenShell, ConsoleText is made available by OpenShell itself as part of its start up process (This is one modification to the standard shell made by the devs). You can then run other tools from OpenShell without issue as long as they do not leverage SimpleInputEx actions if on a Classic MacPro (Discussed later).
If you replace OpenShell with Refit's shell, or any other standard shell, and run this in OpenCore, you will get a blank screen as such shells expect to be running in an environment that already has ConsoleText made available. Refit, or derivatives such as Refind and RefindPlus, provide text mode for such shells when you trigger the shell. OpenShell, as said, activates this by itself.
On the opposite, if you take OpenShell and run it from Refit, Refind or RefindPlus, you will get the same outcomes as with Refit's shell. The environment will have ConsoleText but OpenShell does not need it as it can activate this itself.
Just to add the missing nuance in that there is nothing wrong with OpenShell and it is actually superior to Refit's Shell. The fact that some things cannot be done on it within the OpenCore environment is down to a separate issue ... with our venerable Classic MacPros.
Basically, OpenShell is a
UEFI Shell which requires SimpleInputEx for some actions. SimpleInputEx is however not available on Classic MacPros and trying to use any such command will result in an error message (Without crashing/hanging your computer - This is another modification made by the devs as other shells would cause a crash). The OpenCore devs have decided that trying to provide SimpleInputEx for Classic MacPros is too much effort BTW.
Refit's shell is an
EFI Shell which needs the older SimpleInput as opposed to SimpleInputEx. SimpleInput is available on Classic MacPros. You can therefore launch Refit's Shell from a Mac Bootpicker, where the older SimpleInput has been activated, and use it to its full abilities on a Classic MacPro. You just need to bear in mind that these full abilities exclude certain capabilities requiring SimpleInputEx.
PS: The text mode environment does not work on Classic MacPros when using vanilla rEFInd (as at v0.12.0) if not using a Mac bootscreen capable GPU and you need RefindPlus to get this.