SQL unexpected token error

I have the below section as part of a long query that keep spitting out the error code:

Encountered unexpected token: "(" "("
    at line 188, column 13.

Was expecting one of:

    "&"
    ")"
    "::"
    "<<"
    ">>"
    "["
    "^"
    "|"

code:

AND (trunc(to_date(peef.effective_start_date, 'yyyy-MM-dd'T'HH:mm:ss.SSSZ')) between TRUNC(:eff_mon, 'MM') and TRUNC(ADD_MONTHS(SYSDATE, 1), 'MM') -1)
AND (trunc(to_date(peef.last_update_date),'yyyy-MM-dd'T'HH:mm:ss.SSSZ') < TRUNC(:eff_mon, 'MM'))

I have tried moving a few things around but nothing is solving the issue.

TYIA!