If you fail to do KYC on the exchange, usually they will ask you to complete the missing parts or fail. some exchanges define several stages of KYC, and if you fail at one you can repeat it. and usually the exchange provides a description of the reason for the failure. such as a document photo that is not clear, you can repeat it.
the impact is clear when it comes to withdrawals. If you have already made a deposit, of course you have to complete KYC to be able to make a withdrawal.