https://www.washingtonexaminer.com/news/trump-has-not-built-a-single-mile-of-new-border-fence-after-30-months-in-office
it isn't due to lack of funding:
"The agency said that it had built 51 miles of steel bollard fence with funding that was set aside during fiscal 2017 and 2018. But while the funding was meant both to replace outdated walls and to place barriers where there previously had been none, the government has only completed the replacement projects. The projects to secure areas with no fence are still in the works."