Is this sentence grammatically correct?

[Annecy]

The had kept only some plywood pieces underneath the scaffolding where its tubes come on the floor.

[Calis]

The _ had kept only some plywood pieces underneath the scaffolding where its tubes come on the floor.

First of all, what is the subject of the sentence? What had kept?
Second, while "plywood pieces" is perfectly acceptable, I would say "pieces of plywood."
Finally, the tubes (or pipes) don't come on the floor, they meet the floor.

[Annecy]

Thank you for your swift reply

Sorry for the misspell occurred in the sentence. Can I write as follows.

They had kept only some pieces of plywood under the scaffolding where its pipes meet the floor.

Thank your for your help on this matter

[emsr2d2]

Thank you for your swift reply

Sorry for the misspell occurred in the sentence. Can I write as follows.

They had kept only some pieces of plywood under the scaffolding where its pipes meet the floor.

Thank your for your help on this matter

"Its pipes" is still a problem for me. The individual pieces of metal used in scaffolding are called "tubes". If you mean "where the scaffolding pipes/tubes meet the floor" then I think that's what you should say. You could omit "scaffolding" there and just leave "the pipes/tubes", I suppose. However, when most people see "pipes" or "tubes" they tend to think of drainage.

[susiedq]

Unusual tense to use : They had kept

But if you insist on it, then I'd suggest

. . . .where the pipes met the floor.

[bhaisahab]

Thank you for your swift reply

Sorry for the misspell occurred in the sentence. Can I write as follows.

They had kept only some pieces of plywood under the scaffolding where its pipes meet the floor.

Thank your for your help on this matter

Are you trying to say, "They had only put pieces of plywood under the feet of the scaffolding" ?
Edit: I think you may be saying that instead of having proper welded plates on the bottoms of the scaffolding tubes/poles making stable bases, they had merely put pieces of plywood. Thus making the scaffolding dangerous.