Per documentation, Solve only gives generically correct solutions. This means that if equations are forced upon parameters (non-Solve variables), then solutions forcing those are removed. In this case, as only one variable d was specified to solve for, all others are regarded as parameters. The following variant will give what I think is the expected result.

Solve[{10.109*w - d*200 - 0.788*500 - 1.231*4*Subscript[x, 5] + 
    16.296*101.8 - 1.231*Subscript[x, 4] == 0, 
  101.8 - 4*Subscript[x, 5] - 2*Subscript[x, 3] - Subscript[x, 4] - 
    Subscript[x, 2] == 0, Subscript[y, 1] + w - 500 - 200 == 0, 
  Subscript[x, 5] == 0.563*Subscript[x, 3], 
  Subscript[x, 4] == Subscript[x, 2], 
  1.5*Subscript[x, 4] == Subscript[x, 5]}]

Quick guess -- you are using what you intend to be subscripted variables, but in fact are only using "box representation", in other words, your expression for x_5 is not in fact a Symbol.

Retry using x5 for x_5, et cet.