="("&B2&")"&C2

( is text, and you need to wrap all text in "" when used in a formula - plus you are combining that with (potentially) a number in B2

B1=50
C2=days

B
C
D
2
50
 Days (50)Days

D2=="("&B2&")"&C2

To add that space after the 2nd ) just include it inside the ""...
="("&B2&") "&C2
B
C
D
2
50
 Days (50) Days