If you’re using the Portfolio Post Type plugin, you will need to reset your permalinks. If you’ve already done that and the portfolio URL is still pulling a 404 error, here are some more troubleshooting steps.
For some people, Jetpack seems to automatically turn on their portfolio feature. This feature interferes with the Portfolio Post Type plugin, so you’ll need to turn it off.
Go to Jetpack >> Settings and find “Portfolios” (it should be under “custom content types”, but if you can’t find it, use the search feature at the top of the Jetpack Settings page to help you find it). Make sure “portfolios” is toggled off.
If you have any other portfolio or gallery plugins installed, try deactivating them. Such a plugin may be interfering with the Portfolio Post Type plugin.