Access denied and/or
Error: “C:\Users\YourUser\AppData\Local\Temp\BT\PID3724\BizTalkAssembly\….dll” does not appear to be a BizTalk assembly, and cannot be deployed to the Configuration database. If this assembly is referenced by other BizTalk assemblies, it needs to be installed into the global assembly cache (GAC) of each BizTalk server.

In my case, our company has a specific way of using an AssemblyInfoShared.cs, that only has the version number in it.
I had everything working, then one day, I noticed an issue and deleted my AssemblyInfo.cs, and added the proper AssemblyInfoShared.cs. So I was missing one of the two required files. (Normally, you just have AssemblyInfo.cs, and you dn’t change the using statements in it.)

The trick, in our environment, is that they both must be there.

The AssemblyInfo.cs has the following lines at the top:

using System.Reflection;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using Microsoft.XLANGs.BaseTypes;
using Microsoft.BizTalk.XLANGs.BTXEngine;

It’s probably the 5th line that identifies the project as a BizTalk Project.
So if that line is not there, or you don’t have an AssemblyInfo.cs file with that line, that could cause this issue.