I had a vehicle run into my Building , which caused around 10,500 in damages the insured vehicle insure wants to pay only 6,800 for the damages saying that this is legal to do because the property has depreciation ,I would like to know if this is legal

Generally, for a property damage claim where the property can be repaired (that is, where the repair cost is less than the then-current value of the property, including any depreciation), the insurer pays the reasonable repair cost. So assuming the building is worth more than $10,500 (i.e. it would cost more than that to replace it), the insurer should be paying what it costs to repair. If it is paying less than you believe the cost to be, that is generally because they believe that the repair cost is less than you thought: they need to pay the "reasonable" cost, not the highest possible or quoted repair cost. Depreciation, though, should not be a factor in a repair situation; depreciation comes into play when something cannot be economically repaired, and so you get its then-current value instead.
(In unusual but possible cases, if what was damaged was some discrete piece or part of your building that needs to be replaced, then they can pay you the depreciated value of that part; for example, say that somehow the impact damaged a sidewalk elevator used to take deliveries--if it's current depreciated value is $6,800 but it would cost more than to repair, they could pay just the depreciated value.)
If you believe the insurer is simply not paying as much as they should, based on either the cost to repair (if that is less than the current value) or the current fair market value (including depreciation, if this is a situation where something is destroyed and must be replaced), you can sue the insurer for "breach of contract"--for not honoring its contractual obligation (an insurance policy is a contract) in regards to this claim. If you can prove they are not paying, as appropriate, either the full reasonable repair cost (you'd have to back it up with estimates and testimony from the contractors making the estimates) or current value, you can get a judgment for the additional money.

Generally, for a property damage claim where the property can be repaired (that is, where the repair cost is less than the then-current value of the property, including any depreciation), the insurer pays the reasonable repair cost. So assuming the building is worth more than $10,500 (i.e. it would cost more than that to replace it), the insurer should be paying what it costs to repair. If it is paying less than you believe the cost to be, that is generally because they believe that the repair cost is less than you thought: they need to pay the "reasonable" cost, not the highest possible or quoted repair cost. Depreciation, though, should not be a factor in a repair situation; depreciation comes into play when something cannot be economically repaired, and so you get its then-current value instead.
(In unusual but possible cases, if what was damaged was some discrete piece or part of your building that needs to be replaced, then they can pay you the depreciated value of that part; for example, say that somehow the impact damaged a sidewalk elevator used to take deliveries--if it's current depreciated value is $6,800 but it would cost more than to repair, they could pay just the depreciated value.)
If you believe the insurer is simply not paying as much as they should, based on either the cost to repair (if that is less than the current value) or the current fair market value (including depreciation, if this is a situation where something is destroyed and must be replaced), you can sue the insurer for "breach of contract"--for not honoring its contractual obligation (an insurance policy is a contract) in regards to this claim. If you can prove they are not paying, as appropriate, either the full reasonable repair cost (you'd have to back it up with estimates and testimony from the contractors making the estimates) or current value, you can get a judgment for the additional money.