The Set to Ready for Billing menu option will be greyed out for you because in version 4.94 of the system these two new permission were introduced:
Permission to bulk set Ready for Billing on Timesheets.
Permission to bulk unset Ready for Billing on Timesheets.
These new permissions now control whether or not that menu option is available to a given user, so you any other users who need to use the menu option would need those permissions added to a role or a group that you've been assigned to.
To add the permissions to a role (another user in your organisation who is an Access Recruitment CRM administrator may need to do this for you if you cannot do it yourself):
In the navigation panel, click Options and then click Accounts.
Select Roles and highlight a role that the affected user has been assigned to already.
Click the + (right hand side of the window) in the Role Permissions header to add a permission.
Find and tick:
Permission to bulk set Ready for Billing on Timesheets.
Permission to bulk unset Ready for Billing on Timesheets.
Click the Select button.
The affected users should then be able to use the menu option to set Ready for billing, along with any other users also assigned to the same role. If it doesn't work straight away, they should just close any open Access Recruitment CRM tabs within Access Evo, then reselect Access Recruitment CRM from the Access button at the top left.