You can message Fandom support and ask for discussions to be turned on support.fandom.com/hc/requests/new

I thought that your Wiki should have enabled that option automatically, but if this happens to you, you can report it to the Fandom staff to enable it or fix it.

@Salcay It depends, there are a couple of reasons why the Wiki may not have Discussions enabled. OP only stated they are admin and never said they founded the Wiki and as such the old admins could have contacted Fandom and requested the removal of the Discussions Board, whereas OP could have adopted the Wiki and want them enabled again, or they could have founded the Wiki many years ago as not all older Wikis did have Forums enabled and as such I don’t believe that they could migrate to Discussions when the UCP update happened.

@Grandpa Lo70 You're right. In this case, this wiki is pretty old, specifically from 2009 and maybe that's the reason why this wiki doesn't have Discussions enable.

@Mypbka In that case then the old Forums were probably never enabled and as such it didn’t get Discussions as there was no Forums to migrate during the UCP update. I’m guessing it’s an old Wiki that you’ve adopted based on what you’ve just informed me, so if you want Discussions enabled all you need to do is contact Fandom staff (using the link that Noon provided earlier) and request they enable them on the Wiki that you’ve recently adopted.

@Grandpa Lo70 Yup, I already sent the form to Fandom Support, and yes, it's a wiki that I've adopted recently. Thanks for information anyway!