Have cards replaced banknotes?

Have (credit) cards replaced banknotes?

WhatHow do you thinkfeel about paying by card?

What you wrote is grammatically correct. My version is more sophisticated.

Do you think (that) we have/we’ve replaced banknotes with credit or debit cards?

I think we should have the opportunity to use both methods of thesepayment, because they’re both very useful and important, especially when we travel.

Or: …especially when travelling.

Have cards replaced banknotes?

No change suggested

'Cash' is the more common word for banknotes. Another option is to use the word for a specific currency (if you are talking about a specific currency). Three common currencies in the English-speaking world are euros, pounds, and dollars. Also 'card' usually means the physical credit or debit card. Colloquially it's common to use the word 'tap' to refer to using your phone to pay with card. So when you're checking out at a store a cashier might ask "cash, card, or tap?"

Do you think that we have replaced banknotescash with credit or debit cards?

Is it correct in your opinion?

No change suggested

"Correct" means there is something that can be right (correct) or wrong (incorrect). You haven't stated an opinion to disagree with.

I think we should have the opportunity to use both of these, because they’re both very useful and important, especially when we travel.

'Because' is a conjunction word in this sentence because it joins two independent clauses. Be careful with commas right before conjunctions.

Is it correcta good thing in your opinion?

You could also say, "Is it true in your opinion?"

Is it correct in your opinion?

No change suggested

This seems a bit formal. Something like, "Do you think this is correct?" would convey a more colloquial feel.

I think we should have the opportunity to use both of these, because they’re both very useful and important, especially when we traveltravelling.

Grammatically correct but doesn't flow very naturally. Something better may be, "I think we should be able to use both as they're both very important, especially when travelling".

I think we should have the opportunity to use both of these, because they’re both very useful and important, especially when we travel.

You CAN use "of these", or you can just omit it since it's clear what you mean. Don't forget to put a period/full stop at the end of every sentence.