When getting set up with Solana development you may run into this unhelpful error message:

$ npm run build:program-rust

> helloworld@0.0.1 build:program-rust
> cargo build-bpf --manifest-path=./src/program-rust/Cargo.toml --bpf-out-dir=dist/program

BPF SDK: /Users/matt/.local/share/solana/install/releases/1.8.6/solana-release/bin/sdk/bpf
cargo-build-bpf child: rustup toolchain list -v
cargo-build-bpf child: cargo +bpf build --target bpfel-unknown-unknown --release
error: no such subcommand: `+bpf`

	Did you mean `b`?

The solution for me was to completely uninstall Rust:

$ rustup self uninstall

Even after doing this I still had a version of rust installed on my system, which may have been the cause of the error in the first place:

$ which rustc
/usr/local/bin/rustc

I uninstalled it with brew:

$ brew uninstall rust 
Uninstalling /usr/local/Cellar/rust/1.56.1... (30,734 files, 763.2MB)

$ which rustc
rustc not found

And then finally re-installed Rust using rustup:

$ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

And then compiling the rust program worked without any issues:

$ npm run build:program-rust                                        

> helloworld@0.0.1 build:program-rust
> cargo build-bpf --manifest-path=./src/program-rust/Cargo.toml --bpf-out-dir=dist/program

BPF SDK: /Users/matt/.local/share/solana/install/releases/1.8.6/solana-release/bin/sdk/bpf
cargo-build-bpf child: rustup toolchain list -v
cargo-build-bpf child: cargo +bpf build --target bpfel-unknown-unknown --release
   Compiling proc-macro2 v1.0.27
   Compiling unicode-xid v0.2.2
   ...