I had the same issue today and solved it by solving background service failure issue, if you have verified your age and you still get this error, check if you see a small popup at the bottom of the page that says " Failed to restart background service". after you fix the background service issue you should be able to login. here is the related forum post and there is a very easy solution at the end of the forum posts. You just need to remove “Bridge-Bitfrost’” folder in C\Users\PCName\AppData\Roaming.
This is from another poster originally written here: Age Requirement for Quixel Bridge - #4 by Nikolay11131
fixed my issues! Goodluck all!