Yes. 4xx codes are client errors: The request contains a mistake or can't be fulfilled as sent.

4xx · Client error

HTTP 426 Upgrade Required

The client must switch to a different protocol, named in the Upgrade header.

Example response

HTTP

HTTP/1.1 426 Upgrade Required
Content-Type: application/json; charset=utf-8

{ "error": "Upgrade Required" }

Look up the headers shown here in the HTTP headers reference.

What does HTTP 426 mean?: Upgrade Required. The client must switch to a different protocol, named in the Upgrade header.
Is 426 an error?: Yes. 4xx codes are client errors: The request contains a mistake or can't be fulfilled as sent.