Mounted Crossbows is a level 2 research and it does not require any blueprints in UWE. You should find it under the Defence category. If you do not, maybe you have some other mod you have loaded that changes it?

I have turned all mods off but UWE and still nothing.

OK not only is mounted xbows missing a lot of basic crafting ie bolts are missing as well. I have turned off all mods but UWE and still there missing. So many people sing this Mod praise but it has been nothing but a headache for me and im trying but not to many people read these, know what the issue is or care to help. I have reinstalled the game and the mod still doesnt work properly.

UWE change the entire research system and adds ton on contents. I assure you if that is your only mod, then it is working as intended.

Are you already on tech level 2? You need to have the tech level 2 research table before any of tier 2 items are showing.

Reinstalled the game and workshop. UWE only mod got lvl 2 table and all it shows are patrols walls and guards under the research so there is no way its working as Intended.

Have you researched for its required prerequisite research the Crossbow Bolts? It won't appear on the list before you have done that. And Crossbow bolts research requires Basic Weapon Smithing. And Basic Weapon Smithing requires the Iron Plates research. So you need to have all these researches done before you can research Mounted Crossbows.