I had the same problem 15,000 baht refund sent to my wallet, and I couldn't activate the wallet.
Lazada gave me the answer (only works using the phone app).
Go to "account" then "wallet"
Tap on the amount - don't try to activate.
Choose "withdraw" then complete the deposit/withdrawal form.
I can't remember exactly, but I think I got a OTP from my bank, then the withdrawal was received a few days later.