The Mann-Whitney U test has the following assumptions:
There is one independent variable that is a dichotomous variable, that is, it has two categories. For instance, sex (male/female) or trial (intervention/control). These categories are mutually exclusive.
The dependent variable should be measured on a continuous or ordinal scale. For instance, Likert items fulfill this assumption.
The last assumption of the Wilcoxon-Mann-Whitney test is that the observations have to be independent.
Importantly, your sample does not have to follow the normal distribution. This is the main advantage of the Mann-Whitney U test vs. t-test.
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