EMB-170 Captain reported a track deviation occurred related to a misunderstanding of their clearance. Reporter also stated they encountered wake turbulence in climb from the preceding A321.

Narrative

Lined up for [Runway] 26L. ATC cleared us for takeoff to fly runway heading. The FO (First Officer) read back the clearance. Rotated without event. I called for the FMS NAV (which was my error) and the FO set the LNAV guidance to NAV. During this time; we encountered wake turbulence from the [preceding] A321 and the aircraft had an uncommanded 25-degree roll to the right. I was focused on returning the aircraft to a wings level attitude and began a turn to erroneously comply with the JACCC SID towards MPASS. Once out of the wake; I called for the rest of the takeoff profile. At this point; ATC cleared us to 8;000 and asked if we were given a heading. I told ATC that we were cleared RNAV to MPASS. ATC advised that normally RNAV clearances are not given until [later in the day]. I then asked the FO what he read back to the Tower. The FO replied that he read back fly runway heading; but assumed that also meant to comply with the SID. The Controller gave us vectors to rejoin the SID at JACCC. I had a brief discussion with the First Officer about the differences between RNAV and non-RNAV takeoff clearances and about the appropriate takeoff profiles for each possible takeoff clearance. During this briefing the Controller handed us off to ZTL. No further events occurred. Crew members should maintain vigilance with any abnormal takeoff clearances ATC may give to the flight. I also personally brief the takeoff clearance again before accelerating the aircraft in a challenge-response manner as another layer on the Swiss cheese model; but I failed to do this properly on this flight.