SQL Server 2014 Reporting Services issues
Adding the account you are logging in on your OS will allow for you to connect to SSRS. You are provided with a 403 error message directly related to an access denied. The type of access being denied here is at the database level, mainly for the Reportserver$ database. Your OS login is unable to personify NT SERVICE since it does not have a login on the server and guest account being probably disabled.

Additionally, verify the following:
'C:\WINDOWS\Microsoft.NET\Framework\V2.0.050727\Temporary ASP.NET Files'. (or where ever your .NET framework is installed on your machine)

Right click on the "Temporary ASP.NET Files" and select properties->secutity.

See if the "NETWORK SERVICE" is already part of the "User/Group" section. If not add it by typing "Network Service".
If it's already there then grant the write permissions.