I'm a freelancer, recently I got a client for a mobile app, the matter is I don't know how to write a technical offer for the app, can someone help me please ?
Not sure what you mean by a technical offer. Perhaps this will help.
You write down what you are going to do, how long it will take, what the customer will get delivered at the end of it. This is your schedule of work and should be as detailed as you can manage it.
You then write a quote for how much money you would want to complete the schedule of work. This will show the total amount and the deposit required to start (and any applicable taxes).
If the customer then has a question like 'What framework will you use?' you reply as honestly and openly as you can, but the next time you write a schedule of work, you include that info too. In this way your quotes and work descriptions improve over time.
There is no single way to do this. There is no right or wrong way except in the final analysis of did you get the job or not.
I hope that helps,