Without more details it could possibly be:
Loading the donor client without a donated account Using Forge or custom Fabric directory Using an unsupported launcherFor the latter two, either delete libraries/EMC and reinstall, or you must download the library manually from https://maven.aristois.net/me/deftware/aristois/446/aristois-446.jar and then place it in .minecraft/libraries/EMC/(mc version)/
Otherwise, please include more details and your latest.log file.