Let’s Encrypt only issues certificates to domains that end with top level domain suffixes in the root zone database managed by ICANN. Reserved suffixes like .local are not supported (and are prohibited by the CA/Browser forum), neither are any other private or alternative DNS root suffixes.
Please also note that .local is reserved for multicast DNS by RFC 6762 and should not be used in traditional DNS. Some client operating systems, including macOS and Linux in certain configurations, will fail to connect to such domains if it cannot resolve them using multicast DNS.