I found out this error today.
The UB Megamenu did show up in the theme. But when I went to change Page Cache to use ‘Varnish Cache’ in the Store->Configuration->Advanced->System->Full Page Cache, the UB Megamenu is missing.
We see that you purchased UB SportsGear without extension, it was not packed with UB Mega Menu extension which led to the issue you mentioned.
For all extensions built-in our M2 themes will need separate purchase. Please consider our M2 extension Bundle to get all extensions at a discount rate.