Sometimes it happens that you cannot create any new reservations after a certain time of the day, for example after 10 pm. You can check several settings in Mews to find out why it happens.
Check if there is a restriction in place preventing you from booking these dates. In the new reservation screen click on Restricted and you will be redirected to the restriction blocking it. Contact the person who created the restriction for more information.
Check if you have the employee right
Create and manage reservations. Navigate to Menu > Settings > Property > Employees and see if you have this right. Contact your Manager for giving you this privilege.
Editable History Window (EHW)
If your check-in time is for example at 3 pm and your Accounting and Operational editable history windows are set to 7 hours, you will not be able to create any same day reservations after 10 pm in the evening. The reason is that a night is considered as consumed at the time of check-in and after 7 hours you cannot create any new nights as they will be created in the past according to your EHW. This applies to reservations created manually in Commander or by the guests in Distributor. That doesn't apply for Channel Manager reservations as Mews is just receiving them when they are sent.
To solve this, you will need to extend the two EHW but always check with an Administrator for the reason of the windows being so short.
Note: If indeed the EHW is the reason you can't create a same day reservation, you will have the following message on the Availability section in Commander: No public rates available.