Explanation
The most likely cause of the issue where the fiber connection from a device to a switch is not working is that the TX/RX (transmit/receive) is reversed. When connecting fiber optic cables, it is important to ensure that the TX of one device is connected to the RX of the other device and vice versa. If the TX/RX is reversed, data cannot be transmitted successfully.
References:
CompTIA Network+ Certification Study Guide, Exam N10-007, Fourth Edition, Chapter 5: Network Operations, Objective 5.1: Given a scenario, use appropriate documentation and diagrams to manage the network.

A wireless survey is the first step that a network administrator should do before adding access points to the network. A wireless survey is a process of collecting data about the wireless environment, such as signal strength, channel usage, interference, and coverage. A wireless survey can help the network administrator to determine the optimal locations and configurations for the access points to provide the best possible coverage and performance for the wireless network. A wireless survey can also help to identify and troubleshoot any issues that may cause improper coverage in some areas. https://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless-controllers/116057-site-survey-guidelines-wlan-00.html

The FIRST step the technician should perform to troubleshoot the issue is C. Clean the fiber patch cord connectors in both buildings.
Dirty or contaminated fiber patch cord connectors can cause signal loss and poor network performance, and may even prevent a link from being established. Before trying any other troubleshooting steps, it is important to ensure that the fiber patch cord connectors are clean and free of debris.

The ifconfig eth0 promisc command should be run on the Linux computer to enable promiscuous mode, which allows the computer to capture all network traffic passing through the switch mirror port. Reference: CompTIA Network+ Certification Study Guide, Chapter 7: Network Devices.