If your users are receiving VSCode error message “Unable to write chat.mcp.access because it is configured in system policy.” it may be related to an out-of-date VSCode (see bug report here). If they still receive it after updating, try enabling this setting at the org or enterprise level:
MCP servers in Copilot
If enabled, users can configure Model Context Protocol (MCP) servers (including third-party servers) for Copilot in all Copilot editors and Coding Agent.
Reading:
Social