Please note that, you missed including DRIVE_ID in the query that refers to ID of document library.
I tried to reproduce the same in my environment by running the same query as below:
PUT https://graph.microsoft.com/v1.0/sites/{SITE_ID}/drive/root:/Signing/Izjava.pdf:/content
Response:

As I missed including DRIVE_ID in the query, it added the document to the drive root by creating a folder called Signing with a file named Izjava.pdf inside it like below:

To add a new document inside a certain document library, you need to include DRIVE_ID in the query like below:
PUT https://graph.microsoft.com/v1.0/sites/{SITE_ID}/drives/{DRIVE_ID}/root:/{DOC_NAME}:/content
Response:

The document named Izjava.pdf created and uploaded to document library Signing successfully like below:

To get the DRIVE_ID of your document library, you can query like below:
GET https://graph.microsoft.com/v1.0/sites/{SITE_ID}/drives
Response:
