Yes, Gianluca. I have made that mistake before.

Thanks for trying it out in your PC.

Very strange! It doesn't work in my PC (see below).

I tried for Firefox and Chrome and it didn't work in both browsers (both updated). My PC is Windows 10 Enterprise 64-bit. I also tried it on another PC with Windows 7Enterprise 32-bit. it didn't work too. I also restarted the sessions.

By the way, I didn't sign in Wolfram when I tried it. Does it matter?

My code is quite lengthy. Maybe I post another one (from other people) which I modified from to get mine.

a=Table[{i, i^2 - 3 i + 2 + 0.3 Random[]}, {i, -1, 3, 0.1}]; ListPlot[a]; b = Interpolation[a, Method -> "Spline"]; c = b'; Plot[Evaluate[{b[x], c[x]}], {x, -1, 3}]

Before, the plot consisted of two curves: a blue one for "b" and a red one for "c". Now when the code is run, there is only the blue curve for "b".

By the way, what do you mean by "notebook"? (Sorry that I am a beginner and not familiar with many terms.)

And if necessary, I can post my lengthy code. Thanks.

Can you check the incriminated apostrophes?

Sorry that I can't follow you (on how to do the checking).

By the way, I tried another thing: the following is from the Documentation Center, as an example of derivative:

f[x_] := Sin[x] + x^2;f'[x]

it works even though I paste it into word first and then copy&paste it back to the Wolfram platform, so it seems that Word doesn't plays "smart". Maybe the problem is applying derivative on an array?

And I also tried typing the apostrophe character directly (the one on the left of the enter key, right?), but it still doesn't work.

The apostrophes I used in the past and now should be the same because I copied and pasted the code used before into a Word file and now I just copy and paste it into https://develop.open.wolframcloud.com/objects/wpc-welcome/ThingsToTry.nb again. Thanks for your reply nevertheless.