The closure date of a requisition is specified when the requisition is created, and your organization may have chosen to set a default closure date that falls a certain number of days (e.g. 365 days) after the requisition was created. Once the closure date is reached, the requisition will automatically become closed and read-only. This prevents you from making any further changes to it, including addition/removal of any linked transaction items, and creation of out of pocket expenses and manual prepaid transactions.
Note: Administrators with User Right - Edit Closed Travel Request access will be able to re-open closed travel requests by editing the Closure Date.