AOI pricing may not be available for a variety of reasons, common reasons include:
Your search was a point search and not a bounding box. The supplier for that image does not support AOI ordering (common for free imagery sources like Landsat). The overlap of your AOI and the scene is less than the supplier’s minimum order (varies by supplier, but generally 25 km2). The width of the overlap may not meet the suppliers minimum swath requirements (not applicable for all suppliers, and varies between them).
Most situations that will cause AOI pricing to not be included (other than a point search) will be noted in the search result's ‘annotations’ field if you need to check the specific cause.