Depending on whether you have a namespace or not, a module declaration may or may not require an =. This caught me out today (it's been a long time since I did any F#); I created a new file and deleted the namespace and got this compile error:
Files in libraries or multiple-file applications must begin with a namespace or module declaration
As I had some F# code open in my browser that had module blah = I copied it and continued to get the same message:

In hindsight, the example given in the message doesn't have an = so I should've figured it out, but it actually took much googling and finding a comment on this answer that made me realise the issue.
I think it might be worthwhile calling this out in the message (or ideally, detecting when there is a module declaration with an = that shouldn't be there and giving a specific message). It seems effort is going into improving messages in #1103.