Is the intended meaning of "cannot bear it" in the Hadith, "Whoever makes a vow he cannot bear, its expiation is the expiation of an oath," the inability to do so according to Sharia, reason, and custom?

First: The hadith you mentioned in your question was narrated by Abu Dawud via the chain of Talha bin Yahya, from Abdullah bin Sa’id bin Abi Hind, from Bukair, from Abdullah bin Al-Ashajj, from Kuraib, from Ibn Abbas, that the Messenger of Allah, peace and blessings be upon him, said: "Whoever makes a vow without specifying it, its expiation is the expiation of an oath. And whoever makes a vow to commit a sin, its expiation is the expiation of an oath. And whoever makes a vow that he cannot fulfill, its expiation is the expiation of an oath. And whoever makes a vow that he can fulfill, let him fulfill it." The preservers of hadith have deemed it to be mawquf (stopped at a Companion) on Ibn Abbas, and Al-Albani authenticated it as mawquf.

Second: Whoever makes a vow to commit a sin must refrain from fulfilling it, due to the saying of the Prophet, peace and blessings be upon him: "Whoever vows to obey Allah, let him obey Him. And whoever vows to disobey Him, let him not disobey Him."

As for someone who vows something that is rationally impossible (like vowing to fast yesterday) or customarily impossible (like flying without a device), their vow is not binding, and they are not obligated to fulfill it. The majority of scholars hold that an expiation of an oath is required.

The jurists have distinguished between these matters, holding that what is rationally impossible does not necessitate breaking an oath, whereas what is customarily or legally impossible can necessitate breaking an oath.

In summary: Whoever vows something that is legally, rationally, or customarily impossible, their vow is not binding. Scholars differ on whether an expiation of an oath is required or not, and the aforementioned hadith is an evidence for those who say that expiation is binding.