What does the error 'Trading is blocked in (instrument) for this session as you are a designated person of the company' mean?

According to regulations (PDF), the trading window for a particular security will be closed for the designated persons of the company who are expected to have unpublished price-sensitive information (UPSI). A “No trading window period” is placed on such securities for the designated persons.

During the "No trading window period", no trading of any kind will be allowed, including exiting existing positions. All the open positions need to be exited before the trading window closes. The duration of this period will be communicated to the designated persons by the company and will also be specified on the company’s website. However, the designated person can use Immediate or Cancel (IOC) order type to trade in any other instrument except for the blocked security. To learn more about IOC validity, see What does Day, Immediate and Minutes Validity mean?