What was the one bill that Republicans filibustered in 2020?
TL; DR: None.
Given the total of 328 and the 327 to 1 distribution in John Roberts' tweet, it appears he was referring to number of cloture votes that were taken in the 116th Congress, which ran from January 3, 2019 through January 3, 2021. During this period 328 cloture motions were filed, 327 of which were filed by Mitch McConnell and one on which was filed by Charles Schumer. The Republicans did not filibuster that one bill. The concept of a filibuster by a majority party doesn't make a bit of sense. The filibuster is a process used by a party that does not have the majority of members in the Senate but does have at least 41 members.
The cited tweet by John Roberts is highly misleading for a number of reasons. A cloture vote, which is what Roberts appears to have counted, is a vote to end debate on a bill, an amendment to a bill, a resolution, an executive branch nomination, etc. The Senate is a deliberative body; it in general needs to vote to end debate. That's a cloture vote. Some of those cloture votes only need a simple majority to pass. Others need a 3/5 majority to pass. It is only on this latter category that a filibuster can occur. In this process, a minority party with at least 41 members can invoke cloture rules on those bills that require a 3/5 majority to end debate to prevent the bill from passing.
One reason Roberts' tweet is misleading is that the vast majority of the 327 cloture votes filed by Mitch McConnell were for approval of nominees by the branch. A few others were for budgetary reconciliation bills. Thanks to the nuclear option, cloture votes for nominees and reconciliation bills only require a simple majority rather than a 3/5 majority. A cloture vote on these types of votes that only require a simple majority isn't a vote to end a filibuster. It is standard operating procedure. The minority party cannot filibuster when only a simple majority is needed to end debate.
Another reason is that the majority party in the Senate has many mechanisms by which a bill desired by the minority party can be killed. Minority-sponsored bills rarely make it out of committee, and if they do, the inevitability of a failed cloture vote tends to stop them dead. The one bill for which Schumer did ask for a cloture vote was Senate Bill S.4653, "A bill to protect the healthcare of hundreds of millions of people of the United States and prevent efforts of the Department of Justice to advocate courts to strike down the Patient Protection and Affordable Care Act." Schumer presumably knew that his filing for cloture would result in failure. There is normally no point in the minority party filing for cloture as the vote would almost inevitably be doomed to failure. I suspect that in this one case, Schumer wanted the Republican votes to effectively kill that bill to be on the public record.
One final reason that this is misleading is various ways in which McConnell maintained a very strong grip on the Republicans in the Senate. McConnell filed cloture motions on Republican-led bills that failed. Some of these were hints to fellow Republicans that they needed to modify the bill to make it more amenable to at least a few Democrats. But others were slams against more extreme members of the Republican Senate that the bills they proposed were dead in the water. For example, Senate bill S.1332, "A bill to set forth the congressional budget for the United States Government for fiscal year 2020 and setting forth the appropriate budgetary levels for fiscal years 2021 through 2029," failed to pass cloture with 22 yeas and 69 nays. This was a vote to sustain a bipartisan filibuster against a bill submitted by Rand Paul, and it was a signal that this Republican-sponsored bill was dead.