There are multiple definitions - click on the translation that matches your context.

guarantee means:

bürgen

Verb

partial

guarantee

verb

Again, the distinction between a guarantee and a Bürgschaft can be found in the entry for Bürgschaft

to guarantee a debt to make a formal promise to accept the responsibility for someone's debt if that person cannot to pay it

Banks will not guarantee payments made by cheque over a certain amount.

to make a formal promise that something will happen

We cannot guarantee the amount of work we would be able to give you.