The error message you're seeing refers to a function in the
unite#sources#mru namespace, which means it's probably in a MRU plug-in for unite.vim, most probably neomru.vim.
There's an issue reported there that is very similar to yours: issue #48: "Vim error: using String as a Float", reporting code that works in NeoVim but breaks in Vim 8.1. That issue was fixed by this commit, around 5 months ago.
So, try updating your plug-ins and see if that fixes that issue?
If it doesn't, see if these are indeed the plug-ins you have and try to disable them to confirm whether that solves your issue or not.
If you don't really use those plug-ins (and just got them from copying and pasting from somewhere in the web into your vimrc), consider disabling/removing them, no point in keeping them around if you don't use them...
If you still can't figure it out, you need to post a more detailed question, including more information about your system, which plug-in manager you're using (if any), which plug-ins you have installed, perhaps relevant snippets from your vimrc, etc.