"Please correct before proceeding." - Web.config error

just open web.config and run again.I've had this problem and I resolved by this solution .

I actually resolved this by making sure the Web.config file wasn't an open document inside VS 2015. In other words, close the Web.config file wherever it might be open, and make sure your Team Explorer isn't causing any problems, like an unexpected change or staged change...rebuild the sln and run...

This happened to me today after getting latest. I resolved by closing and reopening the solution, rebuild, then run. All is well...

Close vs2017 and open it again worked for me