When upgrading an SPFx project pre Microsoft Teams support the mkdir command to create the "teams" folder has an incorrect slash in both the FN018001 Web part Microsoft Teams tab resources folder | Optional section as well as the Summary section.

FN018001 Web part Microsoft Teams tab resources folder | Optional
Create folder for Microsoft Teams tab resources

Execute the following command:

mkdir c:\code\myproject/teams 

Environment

Windows 10, VS Code Terminal (cmd), 2.10.0