An exclamation mark on the terminal screen means the terminal is holding data that hasn't synchronised with Adyen. This could be because the terminal lost connectivity, or hasn't been registered under a valid merchant account.
Ensure that the terminal is connected to the internet and that the PSP Diagnostic is successful.
If this is the first time you are using the terminal, then board it following the User manual.
If the exclamation mark does not disappear, reach out to Adyen Support with a description of the issue.