An organization does not have an Account Forecasting Model set up.

For Advanced Account Forecasting, the Start Period represents an offset relative to the current forecast period. Salesforce defines a value of 0 as the current period, a positive value as a future period, and a negative value as a historical period. Therefore, when the current monthly period is February 2024 and forecasting needs to begin in January 2024, January is exactly one monthly period before the current period. The correct offset is consequently −1.

The 18-month requirement concerns the forecast horizon, which is represented through Display Duration, not the Start Period. Start Period controls where the forecast window begins relative to today; Display Duration determines how many periods are generated and shown.

For example, Salesforce documents an Advanced Account Forecasting configuration in which a Start Period of −6 begins six periods in the past. The same rule applied here produces −1 for one month before February.

This relative-offset model is important because period groups are configured dynamically around the current period rather than hard-coded to a fixed calendar month.

Study Guide references/topics: Configure and Build — Advanced Account Forecasting; Period Groups; Start Period; Display Duration; Forecast Horizons.