Arduino expected ')' before '{' token
Welcome to this site, next time use ctrl-k to align your code.
The problem is in:
Serial.print(mfrc522.uid.uidByte{i} &lt; 0x10 ? &quot; 0&quot; : &quot; &quot;);

To get the value of an array, use [i], not {i}, thus
Serial.print(mfrc522.uid.uidByte[i] &lt; 0x10 ? &quot; 0&quot; : &quot; &quot;);

(similar to other lines).
To give some additional hints:

Try to prevent duplicated code by storing the byte into a variable. Now you can use uidValue in the subsequent lines.
uint8_t uidValue = mfrc522.uid.uidByte[i];

Store 0x10 inside a constant.


So you get:
const uint8_t MAX_HEX_SINGLE_DIGIT_VALUE 0x0F;

for (byte i = 0; i &lt; mfrc522.uid.size; i++)
{
    uint8_t uid_byte = mfrc522.uid.uidByte[i];
    Serial.print(uid_byte &lt; LOW_VALUE &lt;= MAX_HEX_SINGLE_DIGIT_VALUE ? &quot; 0&quot; : &quot; &quot;);
    Serial.print(uid_byte, HEX);
    content.concat(String(uid_byte &lt; MAX_HEX_SINGLE_DIGIT_VALUE ? &quot; 0&quot; : &quot; &quot;));
    content.concat(String((uid_byte, HEX));
  }

I assume the Serial lines are only for debugging, otherwise you can combine them too.