When Ngen .DLL got Error System.IO.FileNotFoundException

Iam new to ngen, the reason i try to ngen is that when iam running the program for first time it's so slow then the second time, due to cold/warm startup.

when i try to ngen my dll, it show this kind of error

1. Warning: System.IO.FileNotFoundException: The specified module could not be found. (Exception from HRESULT: 0x8007007E) while resolving 0xa0003d5
2. The specified module could not be found. (Exception from HRESULT: 0x8007007E) while compiling method add_AppraisalCard.button1_Click

iam running it by ngen install class.dll

My question is

1. What does this mean ?
2. How to fix this warning/error ?
3. Is it okay to ignore it ?
4. Where can i found the output of the ngen file ?

PS: my program output is classlibrary (dll) not an exe, running on framework 3.5 and inside my dll contains another dll such as devexpress