Your id does not have package name as usually have android apps.
You have two options:
1 Disable disableIdLocatorAutocompletion as mentioned in link send by @mykola-mokhnach
2. Use UiAutomator search ->
WebElement el = driver.findElement(AppiumBy.androidUIAutomator("new UiSelector().resourceId(\"nextBtn\")"));
// more to read -> https://developer.android.com/reference/androidx/test/uiautomator/UiSelector