An exclamation mark on the terminal screen means the terminal is holding data that hasn't synchronised with Adyen. This is typically due to the terminal unable to connect to the internet to send the data back to Adyen.
Ensure that the terminal is connected to the internet and (you can check this by running a PSP Diagnostic test from the Admin menu on the terminal).
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.