How to debug " Candid interface compatibility check "?

// ... end of my  code :
// new auto did generate: in ic_cdk 0.10.0
use ic_cdk::export_candid;

btwl@btwl-virtual-machine ~/D/i/tax_lint (master)> dfx deploy backend
Please enter the passphrase for your identity: [hidden]
Decryption complete.
Deploying: backend
All canisters have already been created.
Building canisters...
WARN: Cannot check for vulnerabilities in rust canisters because cargo-audit is not installed. Please run 'cargo install cargo-audit' so that vulnerabilities can be detected.
Executing: cargo build --target wasm32-unknown-unknown --release -p backend --locked
   Compiling ic-cdk-macros v0.7.1
   Compiling ic-cdk v0.10.0
   Compiling ic-cdk-timers v0.4.0
   Compiling backend v0.1.0 (/home/btwl/Desktop/ic/tax_lint/backend)
    Finished release [optimized] target(s) in 9.03s
Installing canisters...
An error occurred during Candid interface compatibility check for canister 'backend'.

Generated did file should contain some service interface
Do you want to proceed? yes/No
Upgrading code for canister backend, with canister ID bkyz2-fmaaa-aaaaa-qaaaq-cai
Deployed canisters.
  Backend canister via Candid interface:

the eeror: “An error occurred during Candid interface compatibility check for canister ‘backend’.” is kind of too simple that make me hard to check where goes wrong.
Any good approach to fix that ?

It’s likely that the did file you generated is empty.

Yes, its empty.If i use hand made generate. : cargo build & wasmtime it is something like this :
lake of function body:

# cargo build --target wasm32-unknown-unknown --release --package "backend"      --features "ic-cdk/wasi"
# wasmtime "./target/wasm32-unknown-unknown/release/backend.wasm"  > backend.did  --allow-precompiled

type Result = variant { Ok : text; Err : text };
service : () -> {
  get_caller : () -> (text) query;
  greet : (text) -> (text) query;
  next_id : () -> (nat64) query;
  now : () -> (nat64) query;
  register_user : () -> (Result);

Do you put this generated did file in dfx.json? e.g., candid = "backend.did"