Replace the _deleteListItem function with the following code:
private _deleteListItem(): void { const url: string = this.context.pageContext.site.absoluteUrl+ "/_api/web/lists/getbytitle('My List')/items(1)"; const headers : any = { "X-HTTP-Method":"DELETE", "IF-MATCH": "*" }; const spHttpClientOptions: ISPHttpClientOptions = { "headers": headers }; this.context.spHttpClient.post(url, SPHttpClient.configurations.v1, spHttpClientOptions) .then((response: SPHttpClientResponse) => { if (response.status === 204) { this._operationResults.innerHTML = "Delete: List Item deleted successfully."; this._readAllItems(); } else { this._operationResults.innerHTML = "Delete: List Item delete failed." +response.status+" - "+response.statusText...