The results are just an example of summary(model) of my mixed linear regression model:
model <-lmer(Expression ~ Batch + AGE.Group + Sample.Site +Gender (1|ID) ,data=df)
and then summary(model) it gives me a nice correlation matrix for all variables as in my example above.
Now with a "basic" linear regression model
model <-lm(Expression ~ Batch + AGE.Group + Sample.Site +Gender ,data=df) it doesnt give me a correlation matrix with summary(model) and I just cant find a way to get it since I have non-numeric, categorical variables.