As the data are responses to a Likert-type item with four response options, it's best to treat the data as ordinal. It probably doesn't even make sense to think of the mean of such a variable. That is, you can't take an average, of, say, responses that are "Never", "Rarely", "Frequently", "All the time".
A simple approach would be the Wilcoxon-Mann-Whitney test mentioned in the question. Note that this test is not usually a test of the mean or median, but tests if the responses in one group tend to be higher than in the other group.
You could also use a test of the median, like Mood's median test. But I'll warn you that this test can get funky when you have discrete responses, like you have.
As to why you aren't getting results from SPSS, I wouldn't know. But the test is easy enough in Jamovi or even R or Python.
ADDITION:
You can run the following in R or at https://rdrr.io/snippets/ without installing software.
At least for the data you supplied --- if I got the numbers correct --- here's what I get.
For these samples, the item "p" tends to have higher values than "g" --- more "4"'s, and fewer "0"'s and "2"'s,
Item1_g = c(3,4,4,3,3,4,3,3,4,4,3,4,4,3,4,3,4,0,4,4,4,3,3,4,4,4,3,3,3,3,2,3,2)
Item1_p = c(3,3,3,4,4,3,4,3,3,4,4,3,3,4,4,4,3,3,4,4,4,4,4,4,4,4,4,3,4,3,4,3,3)
wilcox.test(Item1_g, Item1_p)
### Wilcoxon rank sum test with continuity correction
### W = 457.5, p-value = 0.2087
library(rcompanion)
vda(x=Item1_g, y=Item1_p, verbose=TRUE)
### Statistic Value
### 1 Proportion Ya > Yb 0.193
### 2 Proportion Ya < Yb 0.353
### 3 Proportion ties 0.455
Data = data.frame(Y = factor(c(Item1_g, Item1_p),
levels=c("0","1","2","3","4")),
Group = c(rep("Item1_g", length(Item1_g)),
rep("Item1_p", length(Item1_p))))
round(prop.table(xtabs(~ Group + Y, data=Data), margin=1),2)
### Y
### Group 0 1 2 3 4
### Item1_g 0.03 0.00 0.06 0.45 0.45
### Item1_p 0.00 0.00 0.00 0.42 0.58
This additional code will create a bar plots to compare the two groups.
library(lattice)
histogram(~ Y | Group,
data=Data,
layout=c(1,2),
drop.unused.levels=FALSE)
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/pHSe6.png)