What are you compiling?
What line of code throws the error?

The error means that you are using a structure named _CONTEXT (maybe you have a variable of that type) and attempting to use a member named Esp, but that member doesn't exist. If the build works fine in 32-bit then I'd say _CONTEXT is defined differently for 32-bit and 64-bit, and to fix this error you need to determine why you're using Esp and what different member of _CONTEXT you should use to accomplish the same goal.