I struggled a lot to  find out this problem's solution . after 4 5 days research, I come to know the exact solution. Kindly follow all the below steps, definatly it will work for Nopcommerce 4.3 Issue using dotnetcore 3.1 or normal  dotnetcore 3.1 project.
Step1 :  check dotnet version and sdk  using cmd

Step 2:  add the IISUSER or you can also give Everyone permission  to the main folder
Step 3: If it is nopcommerce 4.3 project then configure the path
and paste below folders in bin->debug->netcoreapp3.1
App_Data,Views,wwwroot,Plugins,Themes,logs,Areas/Views,web.config
Note : this is the main solution after adding the web.config under bin->debug->netcoreapp3.1
<aspNetCore processPath="C:\Program Files\dotnet\dotnet.exe" arguments=".\Nop.Web.dll" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" forwardWindowsAuthToken="false" startupTimeLimit="3600" hostingModel="InProcess">

Note : If  you have configured your site already in  iis, then do  the last step
reset the iis using cmd.
open cmd as administrator and run  command  iisreset

I hope  this will resolve your all issues