Reports of the Key Account Manager (KAM) are based on the Key Account Plan (KAP). You may ask your manager to provide you with the KAP. This KAP also includes the format of the reports to be submitted by the KAM. It will help you in preparing the format of the report. If your manager is not able to provide the report, then you may prepare a summary of the meetings that you had with the accounts. Additionally, you may write about the amount of sales that were closed and what is expected in the future.
By the way, it is an error on your manager's part to tell you to prepare the report. It is his duty to provide the format of the report. Juniors are just expected to submit the reports timely. I say this with conviction as I conduct the training program on KAM.
Thanks,
Dinesh Divekar