原因：你的SignalR配置了权限 [Authorize] 特性

   [Authorize]
    public class MessageHub : Hub<IMessageClient>
    {}

解决

builder.Services.AddAuthentication(x =>
{
    x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
    x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
}).AddJwtBearer(x =>
{
   //...
   //上面是你其他代码，下面的代码需要你加入
    x.Events = new JwtBearerEvents
    {
        OnMessageReceived = context =>
        {
            var accessToken = context.Request.Query["access_token"];

            var path = context.HttpContext.Request.Path;
            if (!string.IsNullOrEmpty(accessToken) && path.StartsWithSegments("/messagehub"))
            {
                context.Token = accessToken;
            }

            return Task.CompletedTask;
        }
    };
});