# Troubleshooting

This page lists the most common symptoms when running the SDK Demo App and how to resolve each one.

| Symptom | Resolution |
| :--- | :--- |
| `Unsupported class file major version` on sync | The Gradle JDK is not 21. See Step 2 of the [Quick Start](/en/getnet-toolbox/sdk-demo-app/first-steps-demo/quickstart). |
| `Could not find com.pagonxt.sdk:brazil-hml` | The bundled local Maven repository for the variant is missing. Confirm that `apolo-sdk-local-maven/brazil/<buildType>` exists — for example `brazil/hml` for the `brazilHml` variant. |
| The QR Code still carries the sample credentials | The Compose preview was not rebuilt after you edited the payload. Open `MandatoryCredentialsQrCode_Preview` or `FullCredentialsQrCode_Preview` and click **Build & Refresh**. See Step 4 of the [Quick Start](/en/getnet-toolbox/sdk-demo-app/first-steps-demo/quickstart). |
| Credentials saved, but the warm-up fails and the main menu does not appear | The warm-up status code names the cause: `401` for an invalid `clientId` or `clientSecret`, `404` for a terminal that is not registered, `400` or `422` for the `terminalCode` or sub-merchant values. See the warm-up errors in the [Error Codes Reference](/en/getnet-toolbox/sdk-white-label/reference-sdk/error-codes). |
| The Setup screen does not appear and the app keeps the previous credentials | They persist in the app database after the first save. Clear them with the reset button in the **Home** screen header, through **Settings → Apps → Apolo DevKit → Storage → Clear data**, or by reinstalling the app. |
| App opens, but sale, refund, or printing fail | The terminal is not ready. Check the POS itself and the Platform and Middleware services in [Prerequisites](/en/getnet-toolbox/sdk-demo-app/first-steps-demo/prerequisites). |
| Colors or receipts do not change | They are build-time customizations. Trigger a `reinitialize(…)` — a `shutdown()` and `setup(…)` cycle — to apply them. |

## Next steps

* [Quick Start](/en/getnet-toolbox/sdk-demo-app/first-steps-demo/quickstart) — the build and run steps referenced above.
* [Error Codes Reference](/en/getnet-toolbox/sdk-white-label/reference-sdk/error-codes) — the SDK's result codes and cause categories.