Ensure you are attempting to log in at https://www.clc2s1.usmc.mil
In Google Chrome
Click the 3 vertical dots at the top right
Click settings
Click Security and Privacy
Click Clear browsing data then advanced. Check as many as want but the only requirement is to check Cookies and other site data and Cached images and files. Click clear data.
Once data is cleared, click Cookies and ither site data, turn on clear cookies and site data when you close all windows.
Finally, close all windows and attempt to log in.
If using Edge
Type internet options in the search at the bottom left.
under general tab in the browsing history section click "delete”
have "temporary internet files and website files" and "Cookies and website data" checked
Click Delete again
Go to the Content tab
clear SSL state
apply and ok
Close and reopen your internet browser