You can generate multiple Routes using Garmin Basecamp and download them to your 66s in one go, but I am guessing that you are trying to create and store multiple Routes on the 66s itself - yes ?
Yes that’s right. I create routes on memory map and am trying to transfer and store on 66s. It’s worked fine in the past. For some reason not playing ball any more!
Are, so you are not creating Routes on the 66s itself you are creating Routes in Memory Map and then sending gpx files to the 66s, have you updated software versions on either Memory Map or the 66s around the same time that you had this problem ?
You could try generating the Routes in Garmin Basecamp instead of Memory Map