If the tempo control is missing, Practice mode is set to speed the song up for you.

Practice tempo is not a tempo, it is an increase. The song gets faster by that many beats per minute on every repeat, so you gradually play at full tempo. While it is on, iReal Pro is driving the tempo, so the tempo control is disabled.

To re-enable the tempo control, open Practice in the player controls and drag the Tempo slider all the way down, until it reads Off.