Reasons may vary from breaching a challenge to not signing your agreement or having a pending payout. Failing a challenge triggers a hard breach, automatically setting the trading account to read-only, preventing any further trades from being made on the account. If a Payout is pending, the trading account will be on read only and no new trades will be able to be opened.
Additionally, if you receive your funded account through the 2x Evaluation or Instant Funding challenge, signing and submitting the agreement in the Client Area is necessary to enable trading. Once the agreement is submitted, the read-only status is removed, and trading is enabled.