How do I resolve 'iMessage activate unsuccessfully' issue on iPhone 12 Pro Max?

with an iPhone 12 Pro Max I can’t activate the iMessages , ( its shows me iMessage activate unsuccessfully ) It used always to work fine with me but today I keep trying the whole day everything what they say in YouTube but no result . I still have only something I didn’t try is i didn’t update to the last iOS I have the one before the last one

what do you guys think ?

[Re-Titled by Moderator]

Original Title: iMessage can’t activate