Ok.
The transmission warning should leave a code in the system which can be read by a garage with the correct diagnostic reader, it is very possible that if the battery or charging system is not operating correctly, and the running voltage is not correct, that the electrical system which controls the gearbox, may not be able to operate correctly.
For this reason it is important that the cause of the battery light be investigated and checked first. As you do cover a low mileage, it could be the case that the vehicle battery has failed internally and this can cause the problem you have.
If the vehicle battery has not been changed for 5-6 years, then this may be the problem. But it needs checking out, I would also recommend that if the garage do not know what to do, it would be advisable to find a garage who are confident in carrying out any work needed.
A garage who Know the system will be more efficient.
I hope this answers your question but I want you to be 100% happy with the answer so do please feel free to ask any further details if there is any point which you are uncertain of, by replying to this thread. I am happy to help. Thank you for using Just Answer Steve