The error message indicates that the 'hr.contract' object doesn't have an attribute named 'ressource_calendar_id'. In the formula you are trying to divide the 'ressource_calendar_id' by the 'full_time_required_hours'. If you want to calculate the number of worked days, you need to access the 'resource_calendar_id' instead of 'ressource_calendar_id' and divide it by the number of required hours.

Here is the corrected formula:

result = contract.x_studio_transport_allowance * (contract.resource_calendar_id/contract.full_time_required_hours)

Note that this formula only gives you the amount of transport allowance for a single day. To calculate the total transport allowance for the entire payslip period, you will need to multiply this amount by the number of worked days during that period.