Hello,

if You created new directory in Your wordpress installation root folder (same whete is wp-admin, wp-includes, wp-content) wanting to acccess:

yourwordpress.com/customfolder

And it given 404 or 403 error?

this may help:

a) Make sure that this folder and files in it has proper rights (example 755 folder and 644 files) also owner of the folder and files should be same as owner of standard wordpress files/folders.

b) check spelling errors

c) create index.html or index.php file in your new folder with content "Hello"

d) create .htaccess file and add following one or all lines into it:
RewriteEngine Off
ErrorDocument 401 default