Error "TMC2208 or TMC2209 on Z2 requires Z2_HARDWARE_SERIAL or Z2_SERIAL_(RX|TX)_PIN
Your question is missing important information, but I can try to figure out something. You didn't say what kind of firmware it is. I assume it is Marlin.
Your problem is that there are no Tx Rx pins specified for Z2 (because there is X, Y, Z, E0, E1 by default on the MKS Gen L board)
I assume you want to use the E1 driver for the Z2 instead of a second extruder. In that case, you have to tell the firmware that you want to use that driver for Z2.
Search the source code for file &quot;pins_MKS_GEN_L_V21.h&quot;
There should be E1_SERIAL_TX_PIN and E1_SERIAL_RX_PIN defined.
Rename them to Z2_SERIAL_TX_PIN and Z2_SERIAL_RX_PIN.

In &quot;pins_RAMPS.h&quot; file, find a block of a few lines that starts &quot;E1_STEP_PIN&quot;, and rename E1 to Z2 for all those lines.

It may (or may not) work now. If it doesn't, please somehow post those configuration files.