EDIT: apriltag_ros requires the /camera_info topic to be published. You need a bagfile with that topic in it.

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The error in your question occurs when no image is published to the subscribed topic. According to the wiki of the apriltag_ros package, the subscribed topic is image_rect, which is the rectified camera image. Normally you would use the package image_proc to automatically publish the rectified image according to your camera calibration file. Since you don't have the rectified image, you can simply remap your image topic, see below:

Here is a launchfile for apriltag I've found:

<launch>
  <arg name="launch_prefix" default="" /> <!-- set to value="gdbserver localhost:10000" for remote debugging -->
  <arg name="node_namespace" default="apriltag_ros_continuous_node" />
  <arg name="camera_name" default="/camera_rect" />
  <arg name="camera_frame" default="camera" />
  <arg name="image_topic" default="image_rect" />

  <!-- Set parameters -->
  <rosparam command="load" file="$(find apriltag_ros)/config/settings.yaml" ns="$(arg node_namespace)" />
  <rosparam command="load" file="$(find apriltag_ros)/config/tags.yaml" ns="$(arg node_namespace)" />

  <node pkg="apriltag_ros" type="apriltag_ros_continuous_node" name="$(arg node_namespace)" clear_params="true" output="screen" launch-prefix="$(arg launch_prefix)" >
    <!-- Remap topics from those used in code to those on the ROS network -->
    <remap from="image_rect" to="$(arg camera_name)/$(arg image_topic)" />
    <remap from="camera_info" to="$(arg camera_name)/camera_info" />

    <param name="camera_frame" type="str" value="$(arg camera_frame)" />
    <param name="publish_tag_detections_image" type="bool" value="true" />      <!-- default: false -->
  </node>
</launch>

In your case, you publish /usb_cam/image_raw in your bagfile, but apriltag with the above launchfile wants /camera_rect/image_rect: you can see this from line <remap from="image_rect" to="$(arg camera_name)/$(arg image_topic)" />. This line uses the remap function where you can remap a topic to a different name. So you just have to change the args camera_name to "/usb_cam" and image_topic to "image_raw"- Alternatively you could pass these arguments during launching in the command line, something like: roslaunch apriltag_ros launchfile.launch camera_name:="/usb_cam" image_topic:="image_raw"