@DrJ Not an expert on MongoDb, but your hint is that the yum command-line package-management utility is telling you that something is wrong with the file it's using to assist in the installation.
Verify that you have a file at the location /etc/yum.repos.d/mongod-org-3.4.repo or whatever it's supposed to be using by using the file command, a la:
[email@example.com box]$ file /etc/yum.repos.d/mongod-org-3.4.repo
/etc/yum.repos.d/mongod-org-3.4.repo: ASCII text
…and once you've verified that it's present (is it supposed to be mongod-org-3.4.repo or mongodb-org-3.4.repo ?), compare the first couple of lines in the file to a few other ones located in the /etc/yum.repos.d/ directory and check that you haven't inadvertently introduced spaces or something into the file.
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