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:B: 0.012

Sig: 0.075

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.