One of the most common reasons for this is that you have already entered the ISRC for another version of the track.
Whenever you upload a track to a release, that track is automatically added to your track board.
And even if you delete the track from the release, or if you delete the entire release, the track will remain in your track board.
So if you previously entered the ISRC for your track, and you either deleted the track from a release, or you deleted the release itself, then that version of the track along with the ISRC will still be in your track aboard. And you will not be able to add the same ISRC to another version of the track.
In order to check whether this is the cause of the problem, visit your track board and search for the track title:
When you find the version of the track that already contains the ISRC, you can either:
Delete that version of the track. Once this version of the track has been deleted you will be able to add the ISRC to another track. Please note, however, that it is only possible to delete a track from your track board if it is not already part of a release that is live on the streaming platforms.
Add this version of the track to the release you are working on. When creating a release, you can import a track directly from your track board by clicking here in the release tracklist (Step 3 of the release set up):
Add this version of the track to a brand new release by clicking here in the track details. Doing this will create a new release with the same title as the track, and with the track already included in the release tracklist:
Whichever option you choose, if you do happen to find multiple instances of the same track in your track board, it is always good 'housekeeping' to delete the versions that you do not need.
In most cases, this is the reason why you are having trouble adding in the ISRC code. If that is not the reason, feel free to reach out to customer support.