Ignore first 2 bars on the track, they are intro, and begin to play right after them, where the bass line begins.
Pay attention that the riff has shuffle rhythm. It means that instead of playing each chord flatly one after another, as it looks on the tab, the shuffle rhythm created by playing pairs of chords in the following way: the first chord in the pair is twice the duration of the second chord.
12 Bar Blues Riff 2
Note that at this riff each chord begins at the end of the previous bar and continue to sound at the beginning of the current one (the chords connected with the curved lines on the tab).