See the hosted server installation instructions.
You shouldn't have to disable enforceMod.
Either the client or the server is on the wrong version, or is missing Valheim Plus altogether.
If you have a version mismatch between Valheim 0.214.300@0.9.9.13 and Valheim 0.214.300, that means that the latter does not have the mod correctly installed, and you should go through the steps in the installation instructions again.
If you have a version mismatch between Valheim 0.214.300@0.9.9.13 and Valheim 0.214.300@0.9.9.12, then the latter needs to update Valheim Plus, and you should go through the steps in the installation instructions again
If you have a version mismatch between a client correctly on Valheim 0.214.300@0.9.9.13 and a server on an old version like Valheim 0.214.300@0.9.9.11, but you did already install the newest version, then your issue is probably actually "My server host won't let me use anything but the official release". This is because there is not a community fix that has the version 0.9.9.11. Well, there are some 0.9.9.11s out there that do work if you went through special steps to install it, but if you don't know what I'm talking about, then see the previous sentence.