Hp15c, using STO throws error 3. Did I break it?

[BobVA]

As Harald noted, it's hopefully just a memory allocation issue - if you press g MEM and hold the key down you'll get a display like: 19 46 0-0 (which is the default configuration)

The first number (19 in this case) is the highest numbered storage register available for use (includes the "i" register and count starts are register 0). If that's a lower number than 5, that would explain why you can't store above that. The manual explains how to change this.