The Pharisees and the Sadducees wanted to see a sign from the Lord. But the Lord refused to grant their request because they already had:

• The signs of the weather — a red sky at night meant fair weather in the morning and a red sky in the morning meant foul weather that day; so they knew how to read signs.
• The signs of the times — they had already seen signs that testified to them that their Messiah had come from God (Jn 3:2; 9:32-33). Some of these included his miracles (Jn 5:36), his virgin birth (Is 7:14); his forerunner (Matt 3:3-6); his baptism (Jn 1:31-34); etc. They missed them all. They said his miracles were from the devil (Mk 3), his birth was from fornication (Jn 8:41); his forerunner had a devil (Matt 11:18) and they turned down John’s baptism (Matt 21:24-27).
• The sign of the prophet Jonas — Matt 12:38-40, as Jonas was three days and three nights in the whale’s belly, so Jesus would be three days and three nights in the heart of the earth; both spent that time in hell and both came up (Jon 2:2; Acts 2:31). That’s all they would need to know and yet many of them would deny that (Matt 27:11-15; 1 Cor 5:12)

He also refused because they were wicked and adulterous, just like the generations of Jews that required signs before them:

• Moses’ generation — his generation required three signs to believe that God had appeared to Moses (Ex 4:8-9) and they were wicked and adulterous (Ex 32:1-9, 25). All of the males 20 years and older died in the wilderness for their rebellion against God before reaching the land of Canaan (Num 32:11-13).
• Elijah’s generation — his generation required fire to fall from heaven and consume a watered down sacrifice to believe that God, rather than Baal, was God (1 Ki 18:37-39) and they were wicked and adulterous (2 Chr 21:11-14). The sign didn’t turn them away from Baal (1 Ki 22:51-53).

In this wicked and adulterous generation, signs will be given which the people will believe but they will be counterfeit signs (like tongues and healing signs today) [2 Thes 2:9-10; Rev 13:13-14]. The "ministers" will be working for the devil (2 Cor 12:12-14).