Indirect Storage (STO IND) ELI5 what and why?

[DM48]

I am writing a small program and understand Indirect Addressing, enough to be dangerous. Makes complete sense.

I am getting "Size Error" when trying to run the following:
55 is in the x register and I want to store it IND in register 01.

x: 55

STO . IND 01

Size Error

I then ran - Size 25 ENTER to make sure I had a 25x1 REGS

No code at this point. I'm just trying to do this via keypad.

What's up with the "Size Error"?