Learn Hardcore German: Wir dürfen jederzeit Fragen stellen.

Because dürfen is a modal verb. In a main clause with a modal, the finite modal goes in the second position and the main verb appears at the end in the infinitive:

Subject + modal (2nd) + … + infinitive at the end. Here: Wir (subject) + dürfen (modal) + … + stellen (infinitive at the end).

What nuance does dürfen have compared with können?

dürfen = to be allowed to, permission (like English “may”).
können = to be able to, ability/possibility (like English “can”). For permission, German prefers dürfen. Colloquially you’ll hear können used for permission, but in careful/standard speech use dürfen, especially in questions:
Darf ich eine Frage stellen? = May I ask a question? (preferred)
Kann ich eine Frage stellen? = Can I ask a question? (colloquial, less precise)

Why say Fragen stellen instead of just fragen?

German often uses the collocation eine Frage stellen (“to ask a question”). With a plural, it becomes Fragen stellen (“to ask questions”).

fragen is the verb “to ask (someone)”, e.g., Ich frage ihn.
To say “ask a question,” use eine Frage stellen, not “eine Frage fragen.”
Your sentence emphasizes the act of posing questions (possibly multiple).