approach to sign button just isn't there!
young players from asia, not belong to any club,
just can't be sign.

is there any FIFA rules 'bout this?
really confused..

Are they greyed?

no, they are not.
actually, i could sign them 'bout 2 month ago(in game), but they asked too high.

so i wait until i get promotion, but now the buttn is gone.

They retiring?

If the aproach to sign button is no longer available then that would suggest to me that they have signed a new contract. If you want them then you will just have to put in a regular transfer bid for them.

The game now has a Free signing transfer window, perhaps it's that that is stopping you signing players on frees.

Are the Asian players from more obscure nations by any chance?

I have also noticed this with unattatched players from nations such as Laos, yet with players from a more reputable nation such as China, this isn't a problem.

This never seems to affect players from other continents however.

young player from singapore,
not touched by any clubs,
did't say he will retire at the end of the season

Maybe the rules in Asia are different to Europe and signing youngsters is not allowed?

I tried to get a 16-year-old regen from Brazil (not on a free transfer) and he can only join when he turns 18, which seems to be a specific rule for Brazil.

There may be similar rules for the country (countries) from which you were looking to sign a player.

Rainmaker, a screenshot of the transfer, personal and home screens of the player would help us