Getting this error even if key is set

I'd consider this a bug -- there should be a clear error message. But, you can resolve this by creating a key as per the instruction on the project page:

Signing and/or validating JWTs requires a secret. The JWT module leverages the key module to manage these secrets. Once everything is installed, you will need to create a new key at Manage > Configuration > System > Keys (admin/config/system/keys). Add a new or existing key there.

and

Once you have created a key, navigate to Configuration > System > JWT Authentication (admin/config/system/jwt). Choose the key that you just created in the previous step.

abhishek_gupta1 → made their first commit to this issue’s fork.

@borwickja, i have fixed the issue and created MR. please kindly review the MR

The logic change looks ok, but you changed the code style to be incorrect:

12 coding standards messages
✗ 12 more than branch result

src/Transcoder/JwtTranscoder.php ✗ 12 more
line 261	Line indented incorrectly; expected 6 spaces, found 8
262	Expected newline after closing brace
263	Line indented incorrectly; expected 6 spaces, found 8
264	Line indented incorrectly; expected 8 spaces, found 12
265	Line indented incorrectly; expected 6 spaces, found 8
265	Expected newline after closing brace
266	Line indented incorrectly; expected 8 spaces, found 12
267	Line indented incorrectly; expected 6 spaces, found 8
271	Line indented incorrectly; expected 2 spaces, found 0
271	Closing brace indented incorrectly; expected 2 spaces, found 0
271	Expected 1 blank line after function; 2 found
274	The closing brace for the class must have an empty line before it

Apart from the coding style issues, this MR resolves the problems for me when running PHPUnit tests on custom modules (requiring JWT functionality).