The current campaign error message for CANNOT_MODIFY_FOR_TRIAL_CAMPAIGN can be a bit misleading.
Current error message:
campaignError":"CANNOT_MODIFY_FOR_TRIAL_CAMPAIGN",
"message":"Cannot modify dates, budget or campaign name of a trial campaign."
I believe the message should also include status, e.g.:
"Cannot modify status, dates, budget or campaign name of a trial campaign."
Reference: https://developers.google.com/google-ads/api/docs/experiments/campaign-experiments