You have likely reached the weekly S-Chip limit for this game mode. This is a normal game mechanic, and the limit resets every week. You can still participate in trials to earn other rewards, such as mind chips.
To check your weekly S-Chip limit, navigate to X-note > Event and select either Trials of Themis or Temple of Trials. The limit is displayed on the main page of the selected mode:
- Trials of Themis: Lower-left of the map screen.
- Temple of Trials: Middle of the pre-challenge page.