When a friend chooses to Settle Up with you via Venmo, we bump them over to Venmo to complete the transfer, prefilling the 'to' field with your Splitwise email address. A common issue that arises is that not everyone uses the same email address for both Splitwise and Venmo. If this is the case, your friend's Venmo payment will result in a Venmo invite email being sent to the email address associated with your Splitwise account, and you won't see the money show up in your existing Venmo account.
To claim the payment you simply have to associate your Splitwise email address with your Venmo account, as a secondary address. You may do this from your account settings page on the Venmo website. Once you do this the money your friend sent you should appear in your existing Venmo account immediately. If you need more help, Venmo has an FAQ entry about it at the bottom of this page: https://help.venmo.com/hc/en-us/articles/235170748-Frequently-Asked-Questions.
If this doesn't seem to be the problem and your Venmo payment has gone through on Venmo, but is still marked as pending on Splitwise, it may be a delay. It can take up to 24 hours for a confirmation to get from Venmo to Splitwise. If it has been more than a day and you are still seeing a problem, please contact firstname.lastname@example.org.