Fix Invalid Imei for Mtk Chipset Using Mobile Uncle and Create Imei.bak File Using Computer.
Know What Kind of Device You Have So to Know Which Chipset You Have Just Google It or Download Cpuz App to Know the Chipset of Your Android. If You Have Mediatek Device Then Your Imei Will Be Repair Using This Method If Your Chipset Is Different Go and Search for Your Chipset
for Mediatek Devices Follow Steps
Requirements
Device Must Be Rooted If Not Click Here and Root First
Mobile Uncle Tool for Android.
Mobile Uncle Tool for Pc

Also you can see Features of Root from here.

Follow the Steps and Fix IMEI

1. Install mobile uncle tool in your computer file given above. No need to worry about language just follow the steps
   Dont worry if you didn’t understand the language follow my steps as below:Press Y as shown in Image
2. Now enter the IMEI Number 1 as shown below: (Note: Enter Your IMEI 1 not the one mention below)
3. Now you have to enter the IMEI number 2 as shown below (Note: Enter Your IMEI 1 not the one mention below)
4. Now just press enter and you will see like this as shown belowNow you have successfully created (.BAK) file of Your IMEI 1 and IMEI 2
5. Now search in your computer imei.bak file that generally in Your OS drive.Now you have a IMEI.bak File of your IMEIs
6. Turn on your Android device and install mobile uncle tool .apk given in required files.
   Note you must have root access to your Android device learn how to root from here.
7.  Now you have to install mobile uncle tool as shown below 
8. Give the root permission to mobile uncle tool as shown below
9. 
   
10. Now you have to click on  IMEI backup restore (MTK) as shown below
11. Now you see as below and you have these options not click on  restored IMEI from SDcard (MTK) as you stored IMEI.bak in internal storage
    If you stored on your SDcard click on restored IMEI from SDcard 2
    As shown below
12. Now your phone suddenly reboots and your IMEI is now correctly working…
    Its 100% working method if you have any problem with this method please do it properly or comment below