can i make a bechamel sauce for lasagna a few hours ahead and refrigerate it?

Yes, you can. Just cover the surface of the sauce with plastic wrap before refrigerating so it doesn't get a skin on top. The bechamel will thicken as it chills, so you'll want to rewarm it gently on the stove, and you may want to whisk in a bit more milk to loosen it to the right consistency before assembling your lasagna.