When an exchange shows a withdrawal to your XUMM account has been processed, but you didn't receive it:
- Check if you made the withdrawal to the right account (your XUMM account address, starting with r...)
- Wait a couple of minutes, sometimes senders don't process withdrawals straight away
- Verify your account transactions on an explorer like xrpscan.com / bithomp.com / xrplorer.com by looking up your account address (r...)
If the transaction isn't there, you really didn't receive the funds because the sender didn't send them (yet).
The sender tells you the transaction has been sent and processed, but the XRP ledger doesn't show the transaction. That means the transaction in fact hasn't been sent on the XRP ledger. The XRP ledger obviously shows the one and only truth about XRP transactions.
A destination tag is not required when sending to XUMM, so that shouldn't be the problem, unless if the sender made it mandatory to enter one, in that case, when sending to your XUMM account, you can just enter any random destination tag (as XUMM accounts are non custodial, they don't need a destination tag when receiving funds).
The funds are currently still in the wallet of the sender. The sender may indicate they have processed and sent the funds, but they possibly made a mistake / a system failed while they processed sending the funds. It will be easy for them to verify and correct this, as you can obviously show and prove that the funds have never been sent on the blockchain.
There's now only one way forward, and that is for you to contact the sender, possibly per support ticket to them, informing them that their transaction to you has not been processed correctly. We wish we could do more, but the only step to take and the only one who can fix this is the sender.