The error 403: Forbidden indicates that the web server denied access to the requested resource, which in this case is the PCAP file. One possible reason for this error is that the HTTPS server is not enabled for the device platform policy, which is a configuration that applies to the FTD devices managed by the FMC. The device platform policy defines the settings for the management interface, the SSH access, the SNMP, the NTP, the DNS, and the HTTPS server. The HTTPS server allows the FMC to access the FTD devices via HTTPS and perform tasks such as packet capture, packet tracer, and file transfer. If the HTTPS server is not enabled for the device platform policy, the FMC cannot access the PCAP file from the FTD device via HTTPS. Therefore, the engineer must enable the HTTPS server for the device platform policy in order to resolve this issue. To enable the HTTPS server for the device platform policy, the engineer must follow these steps:

Log in to the FMC web interface and navigate to Devices > Platform Settings.

Select the device platform policy that applies to the FTD device and click Edit.

In the General tab, check the Enable HTTPS Server checkbox and click Save.

Deploy the policy changes to the FTD device and wait for the deployment to complete.

Try to access the PCAP file again from the FMC web browser using the same address.

Alternatively, the engineer can also enable the HTTPS server for the FTD device from the FTD CLI using the command configure network https-server enable. However, this method is not recommended because it may cause a configuration conflict with the FMC123

References := 1: Use Firepower Threat Defense Captures and Packet Tracer - Cisco 2: Cisco Firepower Threat Defense Configuration Guide for Firepower Device Manager, Version 6.6 - Device Management Basics [Cisco Firepower NGFW] - Cisco 3: Cisco Firepower Threat Defense Command Reference - C through D Commands [Cisco Firepower NGFW] - Cisco