Only then can I have

Why it is not a question and the modal verb is before the subject?

It is not a question. The speaker is not asking if he can have progress. The “only then can I have” construction is an example of something called inversion, where the order of the words is reversed to create emphasis.

To say this without the inversion, you would say “I can only have progress if I practice diligently.”

See: Only then can we know

Ok! Galiana!!!
Thanks for help me!