At 6 weeks and 6 days of pregnancy, an HCG level of 1364.5 mIU/ml is within the normal range. However, the non-visualization of the sac during your first scan may be concerning. There could be several reasons for this, such as an early-stage pregnancy, incorrect timing of the scan, or a possible miscarriage. It is important to consult with your healthcare provider for further evaluation and guidance. They may recommend additional tests or scans to determine the cause behind the non-visualization of the sac and ensure your well-being during this crucial time in your pregnancy journey.