First take the capital cities of the countries in order

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

Now notice that

The beginning of each capital city give the chemical symbols for the first $11$ elements of the periodic table in order.

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

So the next country he would have to visit would have to have

Its capital city begin with Mg which does not exist.

First take the capital cities of the countries in order

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

Now notice that

The beginning of each capital city gives the chemical symbols for the first $11$ elements of the periodic table in order.

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

So the next country he would have to visit would have to have

Its capital city begin with Mg which does not exist.

First take the capital cities of the countries in order

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

Now notice that

The beginning of each capital city give the chemical symbols for the first $11$ elements of the periodic table in order.

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

So the next country he would have to visit would have to have

Its capital city begin with Mg which does not exist.

First take the capital cities of the countries in order

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

Now notice that

The beginning of each capital city give the chemical symbols for the first $11$ elements of the periodic table in order.

Hanoi
Helsinki
Lisbon
Berlin
Bogota
Chisinau
Nuku'alofa
Ottawa
Funafuti
New Delhi
Nairobi

So the next country he would have to visit would have to have

Its capital city begin with Mg which does not exist.