Hi, Kushal!
Thank you for visiting the Apple Support Communities. I understand from your post that you would like to use an NTT DOCOMO SIM card you have been using in an iPhone 6 you purchased from them in an iPhone X you purchased through au. I'm happy to help!
While the SIM cards in both the iPhone 6 and the iPhone X are the same size, your iPhone X may be locked to au. If it is locked, only an au SIM can be used in the iPhone X unless and until au authorizes the device to be unlocked for use on another carrier. Only au can unlock the device, and you can visit their website to find out about their unlock policy here: SIM Unlocking Procedures | Smartphone or mobile phone users | au.
Once au authorizes an unlock of your iPhone X, you can follow the steps in How to unlock your iPhone for use with a different carrier - Apple Support to complete the process on your device.
For additional questions, please get in touch with au directly: Contact information for contract, etc. | Support | au.
Have a wonderful day!