It might be due to technical or operational issues. Technical issues involve the cash out setup. Either, the bookie does not offer that service, or there is something wrong with its configuration. Confirm if your betting site offers the cash out feature.
If it is operational, then the timing of cash out is the problem. Check on the terms and conditions for a specific time when the feature is available. Also, not all games can access the feature. Confirm from the betting site if the sport offer cash out service.