Hi @JB199zero-5045 ,
The name of the vNet is VNet100? It's selectable. The bold text RedTeamRG is the name of the resource group the vNet is related to and this is not selectable as a vNet.
After selecting the VNet100 in the Virtual Network field you should be able to select a Subnet of the vNet VNet100.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten