What is the problem?
Not queuing T0, that tool doesn't exist according to the printer profile or was reported as invalid by the firmware. Make sure your printer profile is set up correctly.
What did you already try to solve it?
Have you tried running in safe mode?
Did running in safe mode solve the problem?
serial.log or output on terminal tab at a minimum, browser error console if UI issue ... no logs, no support! Not log excerpts, complete logs.)
Additional information about your setup
OctoPrint version, OctoPi version, printer, firmware, browser, operating system, ... as much data as possible