Userform multi-tab error message

[exceljunior94]

Hi guys, hope everyone is having a great day, i need help with regards to my multi-tab userform on vba excel.
I currently have 6 tabs and each tab there are are about 5 combo boxes, and a NEXT & CANCEL command button. I want a code that if the user forgets to click on one of the combo boxes, there is an error message asking"please complete the form", and also prevents the the NEXT button from going to the next tab.

If you could help me urgently, much appreciated

thanks