Can I power Arduino with 12V 1.2A adapter
If it is an Arduino Uno Rev3 SMD, the documentation (see Juraj's comment) is of relevance here, click the tab &quot;Tech Specs&quot; to find:

Operating Voltage     5V
Input Voltage (recommended)   7-12V
Input Voltage (limit)     6-20V

While the microcontroller's (ATmega328P) operating voltage is 5 V there is an on-board voltage regulator to make the board work within the input voltage range of 6 to 20 V (max limits) or the recommended range of 7 to 12 V.
Operating this Arduino with a 12 V power adapter is within spec and therefore safe to do. The current rating of the adapter of 1.2 A is also more than sufficient to run the Arduino.