I'm really new to coding and to R so struggling to find an answer I can understand, or recognise if it is an answer. I'm hoping someone(s) can help.

I'm trying to conduct a multiple linear regression in R.

The data is from a survey of 561 people. Here's a sample of the first 12 lines of data.

I want to look for the significanc between a positive attitude, which is a mean of 4 Likert-scale questions (dependent variable), and the explanatory variable of age, gender, user and format.

I carried out the code all in one:

mlrpositive.lm <- lm(mean_pos ~ age + gender + format + user, data = mlr.data)

I noticed it was missing explanatory variables, for example in age it was missing '18-24', gender was missing 'female', user was missing 'no', and format missing 'Audio'.

I carried out each one individually and got the same result.

positive.gender.lm <- lm(mean_pos ~ gender, data = mlr.data)

I don't know how to correct this.