can I use my chess account on another device?

You should be able to. 

Yes, you can. Actually you should, as having more than one account is not allowed.

Do you see any error message when you try to log in with your ipad?

If i purchase the platinum membership on my phone. if i log in the account will i have platinum membership?

Yes, the membership is linked to your account not your device

I'm having the same trouble as catmummy. I can't get my phone and computer to sync to the same account. I signed up on my phone for the gold level app. I've been using the free-week option on my computer, so . . . now there are two of me, and I can't seem to fix it.

Sylvex22, (also known as) sylvex1

Two usernames means you have two accounts.

You can't sync two separate accounts.

I signed up with chess.com on my iPad but cannot access it on my phone. I am a Diamond member. Can I use my another device?

I signed up with chess.com on my iPad but I’m getting a new iPad can I log into this account I am a diamond member

If you used a username/password on the old device you should be able to use the same. If you had it connected via the iOS account you would need to use the exact same account