RISC-V registers are named x0, x1, ..., x31. RISC-V assembly has various symbolic names for those, such as

• ra for return address
• sp for stack pointer
• t0, ..., t6 for temporaries

etc. The symbolic name fp for frame pointer (which you find in some RISCV assembly descriptions) is not recognised. When using in e.g. lw a0, 4(fp) a

  "fp": operand is of incorrect type

is returned. I don't know if this is a bug or not.