After a successful upgrade to magento 2.4.2 from 2.3.3, the gene/braintree module was deleted and replaced with Braintree Core, and I noticed my old settings were retained. I cannot get checkout to initiate, when I click on Paypal or Credit Card (braintree payment options) I get "Sorry, but something went wrong. Please contact the store owner.". I have created new API keys and verified that my merchant ID is entered correctly, nothing seems amiss in settings. Has anyone else encountered this?
There are many cases depends on a given issue. First do check the cache
Open the site different browser private mode. or Open browser with disable cache Inspect -> Network -> tick a "Disable a cache"
If any browser Paypal or Braintree show without error. It may browser cache issue
You should disable the any cache like cloudflare or varnish.