The certificate thumbprint is configured in the website's configuration file, located at <Website Root>/App_Config/Y.Commerce.Engine/Sitecore.Commerce.Engine.Connect.config (or in a custom patch file created for your solution) and in the Commerce Engine environments under <Commerce Engine Root>/wwwroot/config.json.
If you have configured a valid thumbprint that contains lowercase letters, for example 2700da6ab17c56a01f6d0762b76b3ca77933a68a, this will trigger the following errors in the Commerce Engine logs.
[20:13:57 ERR] ClientCertificateValidationMiddleware: Certificate with thumbprint 2700DA6AB17C56A01F6D0762B76B3CA77933A68A does not have a matching Thumbprint. [20:13:57 INF] ClientCertificateValidationMiddleware: Certificate with thumbprint 2700DA6AB17C56A01F6D0762B76B3CA77933A68A is not valid.
This is caused by the logic used to compare the thumbprint values. The thumbprint in the Sitecore configuration file is transformed to uppercase while the thumbprint from the Commerce Engine configuration is not, so when the case-sensitive comparison is performed the result is a mismatch.
As the thumbprint is not case-sensitive, you can safely update the thumbprint values to be uppercase to resolve these errors.