It is normal to sometimes get this warning when flying close to the aircraft’s limit. It just means you need to fine-tune your options.
The Boeing range is calculated under ideal conditions. The actual range can change day-by-day and depends on many factors, such as upper winds, air temperature, passengers/cargo, cost index, how far away your alternate is, how much extra/contingency fuel you require, etc. It is normal for the actual range to be slightly lower than what is advertised in Boeing marketing documents.
You simply need to do some manual tuning to make this flight possible. The fuel deficit is quite small so you likely only need to make a small change, for example:
- Planning less passengers (your OFP had planned 156, if you set 149 the flight becomes possible)
- Planning less contingency fuel (your OFP had set 5%, you could use a fixed value of 15 minutes instead)
- Choosing a different alternate airport (for example SUMU is closer and requires less fuel)
Having to fine-tune a flight plan when close to the maximum aircraft range is perfectly normal and not an indication of a bug.
Best regards,