When I try to signup for a Google Cloud free trial, I get the following error after the page which checks your details and payment method: An unexpected error has occurred. Plea
As stated in the documentation, to sign up for the 12-month, $300 free trial, a credit card or bank account is needed so Google can verify your identity. You will not be charged or billed during free trial.
Here is the link where you can check this information.
My old address was listed on my payments profile. I created a new profile with the correct address and this solved the issue.
[After going around in circles, it appears my old address could only be updated by contacting Google directly despite their online instructions to the contrary. Instead of waiting to speak to Google, I just created a new profile.]
The issue is likely that you are using a prepaid card, which G Cloud unfortunately does not accept.