This happened to me again yesterday (26 Apr 2020) with VS for Mac. I tried a few things including restarting my machine and couldn't connect and was getting the failed to sync error popup... I tried again this morning to add account and it didn't work. Heres what i did that worked for me.
I closed VS for Mac and then deleted the IOS project obj/bin folders. I then disconnected my physical iphone from my mac. Started VS for Mac and then plugged in my iphone. I then went through the process again clicking Visual Studio/Preferences/Apple Developer Accounts/Add Account and it finally worked this time after several tries before doing this.
trucifer
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**[FINALLY GOT THE FLASH DRIVE TO WORK]**
So I kept getting this error (Volume timed out while waiting to eject) while trying to eject the flash drive using terminal (diskutil eject disk3). The disk was not showing up in finder and was "grayed out" in disk utility. I ran first aid on the flash drive using disk utility and got an error. I thought the flash drive was done eventhough I just bought it on Amazon a few months ago (256 GB mini ssd).
So i kept trying to eject it using terminal (diskutil eject disk3) and after several tries it finally worked! I then reformatted it and it seems to be good now, I can now see it in finder. I will report back..
paste this command in terminal: diskutil list
Identifed my target disk as disk3
I then ran this command to eject the disk: diskutil eject disk3
Like i said after several tries, it finally worked and the disk ejected. I was certain that the disk was bad
Big Sur and flash drive problems here too. Has anyone else noticed these forums are just a place to leave a comment about a issue- can't say i have ever came here to apples forum and found a solution to whatever problem I was having...
I have the same problem. I was able publish visual studio IOS app with no problems. I installed a visual studio update the other day and all of a sudden i got a compatibility error that required me to update to the newest version of xcode. I downloaded the new version of xcode and Now when i try and publish my app i get the invalid toolchain error that you got... Did you fix your issue?