You need to ensure that you’re using the 16-digit ID available here .
The DP Name is Zerodha Broking Limited.
Should you see an error ‘Invalid DP ID/DP account’ you can split the demat account number (16 digits) as DP ID: 12081600 and DP account: the remaining 8 digits.
Alternatively, you can apply via your Bank’s Net Banking ASBA portal as well. The process is similar to process of applying for IPO, if you don’t know, you can this post to know the process.