How do I approach a linear mixed effects model with a 2-level group?

Hello @s75moham, welcome in this group.

I have no experience at all with 'linear mixed effect models' but being interested I tried to reproduce your problem. However I run into errors such as object 'Subject' not found , but maybe I am not using the same packages as you. Therefore I enclose a REPRoducible EXample (reprex) of what I did.
A reprex consists of the minimal code and data needed to recreate the issue/question one is having. See e.g. whats-a-reproducible-example-reprex-and-how-do-i-do-one:


my_data = tribble(
  ~Subject, ~Group, ~Value1, ~Value2, ~Outcome,
 1,        "A",        12.404,    5.00,     1.33,
 2,        "A",        14.898,    4.34,     6.77,
 3,        "A",        13.870,    2.53,     5.01,
 4,        "A",        14.313,    9.33,     6.82,
 5,        "B",        13.407,    4.99,     8.42,
 6,        "B",        13.480,    2.59,     9.11,
 7,        "B",        13.761,    1.42,     8.21,
 8,        "B",        12.607,    0.55,     8.32

lme(Outcome~Value1+Value2+Group, random=1|Subject, data=my_data)
#> Error in reStruct(random, REML = REML, data = NULL): object 'Subject' not found

Created on 2020-09-03 by the reprex package (v0.3.0)

I would be helpful if you present your problem in the same way.

By the way 'googling' on linear mixed-effects models using r also produced some results, but probably you are already familiar with these.