My esim isn't activated in mobile data how can I fix this problem

First do this, and if it doesn't work contact your esim provider

1.    Settings, Apps, Messages,

2.    turn off iMessage   

3.    Settings, Apps, Facetime,

4.    Turn off Facetime, 

5.    Power down the phone

6.    Turn on the phone again 

7.    Settings, Apps, Messages, turn on iMessage,   

8.    Settings, Apps, Facetime, turn on Facetime

Did you migrate from an older phone?

If so, did you transfer the sim over from the older phone?

Do you have your carrier's / Network's app on your iPhone, if so, can you set it up from there?

If no, then you may need to contact your carrier / Network and have then set it up or send you a QR code to activate service on your device.

click here ➜ Set up cellular service on iPhone - Apple Support