Presumably your tab staff is set to one of the styles that does not display rests at all. In order for rests to display, try the "Full" instead of "Simple" or "Common" style in Edit / Instruments. You can also right click the staff and go to Staff Properties / Advanced Style Properties to toggle display of rests.
If you continue to have trouble, please ask for help in the Support forum (eg, "Help / Ask for Help", from within MuseScore).
Comments
Presumably your tab staff is set to one of the styles that does not display rests at all. In order for rests to display, try the "Full" instead of "Simple" or "Common" style in Edit / Instruments. You can also right click the staff and go to Staff Properties / Advanced Style Properties to toggle display of rests.
If you continue to have trouble, please ask for help in the Support forum (eg, "Help / Ask for Help", from within MuseScore).