Google typically won't tell your what the problem is, so you need to work it out first. Don't appeal if you are simply saying "there is nothing wrong". That is a waste of an appeal, and you might only get one or two goes.
The most common reasons for a suspicious payment suspension:
- Your card was used in another Google Ads account. If that account was suspended, you won't be able to fix this. If this is the reason - don't appeal yet, ask for more advice.
- The card holder name is different to Google Ads account billing data.
- It is a virtual card or from a service known to be used to avoid scrutiny
- Spelling mistake in the billing details
- Multi-country scenarios - where you are, your billing details, where the account was opened, the currency of the account, are different countries. Even with COVID, Google finds this suspicious
- Using a VPN
In the appeal send proof, like photos of documents or ID. Explain succinctly why everything is legit. Don't ramble or complain.