i m curious as to why cannot beams be fired manually from capital ships towards the target,
Because they can't. Beam weapons, in BP, are streams of Plasma encased in a magnetic bubble. Said magnetic bubble is somewhat susceptible to outside interference, and while natural interference can be compensated for, the artificial jamming coming from a UEF AWACS is not that easy to circumvent, unless you manage to get FTL readings on the jammers. Which is why that unlucky AWACS whose name I keep forgetting from Delenda Est had to be tagged before it could be targeted.
Slash beams can get around that by arcing through the jammers' area of influence, and because their containment fields are much stronger than those of direct-fire beams.
and how those Ewar jammers work?
Very well, thanks for asking.
and on another not so BP question, why capital ships cannot or don't have any shields?
In universe, capital ships can't have shields due to exponentially rising energy requirements. In other words, the energy required to project a full shield bubble around a capital ship is several orders of magnitude above what would be required for a fighter; and while in BP canon capital ships have shield generators layered into their armor at critical spots, it's still problematic to keep all these systems up and running while also powering beam cannons.