You can also run G32 before the G29 to set the z=0. Just what ever method you chose, make sure you do it the same way each time. It was a change some time after 2.02 they added that warning.
From my understanding the idea is that before the mesh starts Z=0 needs to be established by some method. The three that I understand are:
G28 (sets the high by homing and then the firmware pulls the height from your printer config) - This may be the least accurate depending on what type of end stops you use.
G30 (sets the Z=0 by doing a single probe) - The guides for Cartesian printers tend to talk about doing it this way and it will work but most Delta printers back off their endstops and that play can make this less accurate. If you use this please make sure you disable this while probing per https://duet3d.dozuki.com/Wiki/Calibrating_a_delta_printer#Section_Homing_the_machine
G32 (Delta Auto Cal sets many values but one of them is the Z=0) - This is what I prefer and seems to be most common among people using Deltas. If you are using S6 and have 9 to 16 points the accuracy is high and it doesn't take long. At the end of the bed.g you can either probe your mesh or load it and the error shouldn't occur.
One way to see if it is the order you are doing things is if you post your bed.g file as I presume you are getting the error after running a G32?