You have a table "Coefficients".
From it you can see that you have:
Predicted Cr measurement = 2.847 + 0.012 * Concentration
In particular for the line related to Concentration, you have:
Since Sig > 0.05, the statistical conclusion is that you have insufficient proof to be able to say that the Cr measurement depends on the Concentration (in a linear regression).
Let's take a look at a graph of your data.
I can see that around 200-250 there seems to be some effect.
Below 180 and above 270 looks like random noise.
I guess that should match expectations.
There is a concentration that gives an optimal absorption.
Lower or higher concentrations will give less effect.
For something like that, a linear regression is the wrong test.
I guess you'd need for instance a non-linear regression test for that.