Ok, i got it! I could enter mnemonic phrase in the app. It won't work before. I will try to send some mbtc to another wallet frome phone. Thanks.

I can't send funds from the app on my phone too. Yes, i have a phrase that includes 12 words.
Name of file is "electrum-private-keys.json".

Hi! I did it on my phone in mobile app "Electrum". I have had seed, master key and private key in file .json. And i can see my funds in it.

Writes that the wallet is for viewing only. What does it mean that my savings will remain there forever?
I created a standard wallet, but did not know that it was view-only. In the application on the computer, the "wallet - seed" button is not active. I have a file with master-key.json, but when I import it, the "Next" button is not active.