Make sure you meet the minimum requirements for SQL and EPO from the Installation Guide for 5.1:
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We had that too. It means you've got some extensions that aren't supported. You'll either have to upgrade them to a supported version or remove them. See McAfee KnowledgeBase - ePolicy Orchestrator 5.1 supported products
you need to review all extensions installed in 4.6 and see if they are compatible with 5.1; I remember during installation a message is showed with what products are incompatible, it is showed by UpgradeCompatibility tool.
Upgrade all extensions and software required, and install ePO 5.1 + L2 + hotfix 1