I will apply for Canada citizenship within few days, but suddenly I noticed I didn’t not find my previous Bangladeshi passport. In citizenship form u have to write last 5 years passport information, my missing information passport 2017-18, so what can I do now
This does not disqualify you. As others have observed, you can and should include a supplemental statement explaining you are not submitting a passport, including as many precise details as you can including the date the passport expired (at least best estimate).
The extent to which it can be a problem depends on how relevant it could be, which is mostly about the extent to which it could have been used during the last five years.
If that passport expired in 2018 and if that passport was replaced/renewed BEFORE you landed and became a Canadian Permanent Resident, there should be no problem at all. Again, include a supplemental statement explaining you are not submitting a previous passport, including as many precise details as you can including the date the passport expired (at least best estimate), and then it is not likely there will be any issue at all.
Otherwise, if you have been in Canada for well over the 1095 day threshold since that passport expired, odds are good it will not have much affect on how things go, but of course without a copy of it there is no confirmation of the date it expired. Nonetheless, the date your current passport was issued should at least help corroborate when it replaced the now missing one. As
@Seym suggested, in this situation being sure to have a good buffer over the minimum 1095 day required presence is a good idea. That's a good idea anyway, but for an applicant with a missing passport, a bigger buffer is a good idea.
As
@wink suggested, you could wait to apply until sometime next year when it has been more than five years since that passport expired. For sure works. But that is probably overkill, as in not necessary.
Note: if that passport was still valid when you applied for and became a Canadian Permanent Resident, and it could have been used within the last five years, you should be able to submit a copy of at least the bio-pages from what you submitted in the course of applying for PR. If you do not have a copy of what you submitted when you applied for PR, probably time to make a more concerted effort to keep and maintain important personal records. Going forward. Like keeping a complete copy of what you submit in making the citizenship application.