If you're thinking about skills:
* Incompetence: This implies a lack of skills or ability, the opposite of having a wide repertoire.
* Inability: This is a more specific term referring to a lack of the specific skill or ability needed.
If you're thinking about performance:
* Novelty: The opposite of a well-rehearsed repertoire might be something completely new and improvised.
* Unfamiliarity: This could mean a lack of experience or knowledge in a particular area.
If you're thinking about the collection of works:
* Single piece/work: This would be the opposite of a collection of works.
* Novel/original: This could be the opposite of a repertoire in terms of being brand new and not established.
Ultimately, the best opposite for "repertoire" will depend on the specific context and meaning you're trying to convey.