Portugal had won their first three matches in the competition but couldn't find a breakthrough against the Scots at Hampden Park in Glasgow on Tuesday (Wednesday AEDT).
Ronaldo, sitting on 133 international goals, had a few good chances to break the deadlock. He seemed upset with the referee after the game ended, leaving the field with sarcastic applause, waving his arms and with thumbs down.
Portugal had beaten Croatia, Scotland and Poland in Group A1, which they still lead with 10 points, three more than Croatia and six more than Poland.
Last-placed Scotland stayed on one point. The home side came into the match on a nine-match winless streak and had lost four games in a row.
"There was a lot missing for Portugal tonight," midfielder Bernardo Silva said.
"We lacked intensity, didn't attack at the right moments. We leave here frustrated."
The top two nations in each group advance to the knockout stage.
At the Nuevo Arcangel stadium in Cordoba, Alvaro Morata scored about 10 minutes after missing a penalty kick as Spain comfortably defeated Serbia 3-0 to secure a spot in the quarter-finals after their third consecutive victory in the Nations League.
Spain's Alvaro Morata missed a penalty before scoring a goal in his side's 3-0 win over Serbia. (AP PHOTO)
Morata sent his penalty over the crossbar in the 54th minute, then found the net for Spain's second goal in the 65th.
"I missed a penalty but ended up scoring," Morata said. "We are in the knockout stages again. It seems easy but it's not, we need to give value to our achievements."
Aymeric Laporte had opened the scoring five minutes into the match for Spain, the defending champions who opened with a 0-0 draw at Serbia, then beat Switzerland and Denmark in Group A4.
Spain lead the group with 10 points, three more than Denmark, who drew 2-2 at last-placed Switzerland.
Serbia are third with four points from the draw against Spain and a win over Switzerland, who remain winless.
Serbia played a man down from the 76th minute after Strahinja Pavlovic was sent off for a sliding tackle. Alex Baena scored Spain's third goal from the ensuing free kick.
Croatia scored three times in the first 26 minutes of a first half against Poland that featured five goals.
The Croatians led 3-2 going into the break in a game that ended in a 3-3 draw in Poland in Group A1.
Piotr Zielinski opened the scoring for Poland five minutes into the match, before the visitors took command with goals from Borna Sosa in the 19th, Petar Sucic in the 24th and Martin Baturina in the 26th.
Nicola Zalewski brought Poland closer in the 45th minute, and Szymanski levelled the match in the 68th.
Croatia saw goalkeeper Dominik Livakovic sent off in the 76th minute for an ugly foul on Poland striker Robert Lewandowski outside the penalty area.
Livakovic kicked the ball clear but kept his foot high during the follow-through and hit Lewandowski just below the knee on his standing leg.
Croatia are in second place in the group with seven points, three less than Portugal.
Poland are third with four points, three more than Scotland.
In other matches, Kosovo beat visiting Cyprus 3-0 in League C, Romania won 2-1 at Lithuania, and Belarus drew 1-1 with Luxembourg at home.
Northern Ireland routed Bulgaria 5-0 at home with a hat-trick by Isaac Price.
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