How to Fix Sim card not activated or message couldn't send in Mi or any device

why the MI (xiaomi) show's the sim card is
not activated, message couldn't send, message couldn't sync

==>Because this Error is MI(cloud services) error cloud mean where your photos, contact, call logs, location, calander, recordes and your message upload in the cloud which just like bakup tool when your phone get factory reset or some time you delete all things from your phone then cloud will restore all things in your phone which is bakup you

But in MI phone when your bakuping your message or enabling message bakup service then MI try to check sim card is actually activated or not so Mi try to send a hidden verification message 

if the message successfuly send the service get enable 

but if the message couldn't not be send so it can't be enable and error like that

Main thing is message is not sending 

you try to send anyone it will show message couldn't send or faild error

Fix
For the enable to send message to anyone you need SMSC number so lets see how to enter or get smsc number
Now i will show 3 ways to enter SMSC number
1. From Mesaage application 
 open Message app >>Go to the settings >>  go to additional setting >> you will see there SMSC (SHORT MESSAGE SERVICE CENTER)>>
like that

Now open the google and search smsc (sim card name) number 

here are some link of smsc sim card numbers

1. Idea

2. Vodafone

3. Jio

4.

2. From Setting About section (tested on only MI PHONES)

Open your android phone settings>> Go to About device or about>> go to all spec>> click on internal storage (7 times) >>
it will open phone information
it will show
phone information 1 (sim card slot 1)
phone information 2 (sim card slot 2)
wifi information

only click on which Sim card is not working
then you will be see there SMSC

ENTER CODE AND CALL

*#*#4636#*#* and then call 

Now you need another android phone who have same sim card as like your then enter code which is i give you then get the number and send it in your phone and write the number and then update it
like that

WARNING IN MY CASE MY SIM IS IDEA 

AND I AM IN MUMBAI 

AND EVERY SIM CARD HAVE DIFFERENT NUMBER BY DIFFERENT STATE

Sometimes in some cases the mi cloud option can't get enable / can't get turning on. So for those who get issue like that.

Here is fix 

Go to the settings>> connection sharing>> scroll down ⬇️>> click on network reset

⚠️BEFORE NETWORK RESET BE MENTIONED THAT IN NETWORK RESET YOUR ALL SAVED WIFI PASSWORD, PAIRED BLUTOOTH DEVICE GET DELETE/REMOVED⚠️