A better version of your sentence is this:
Er ist ein talentierter und intelligenter Mann, aber wegen seines mangelnden Selbstvertrauens hatte er sich nicht um diese Stelle beworben.
So, in this case it is one of those:
schwach ausgeprägtes Selbstvertrauen
The noun should be »Selbstvertrauen«, not »Selbstwertgefühl«. Later means something that doesn't fit very well into this sentence:
This literally is: self-value-feeling. It is what you think of yourself how valuable you are. If you think that the value that you represent is high (i.e. if you think you are a valuable person), than you have a high Selbstwertgefühl. If you think you are worthless, you have a low Selbstwertgefühl.
This is literally: self-trusting. This is how much you trust in you own capabilities. If you think that you can do well whatever you should do, then you have a high Selbstvertrauen. If you think you will fail before you even tried, then your Selbstvertrauen is low. This often correlates with Selbstwertgefühl, but is different from it.
There is another word that is similar to those:
This is literally: self-respect. This is how much respect you have of your own person. This is very close related to Selbstwertgefühl, but not exactly the same.
now for the attribute:
Your Selbstvertrauen can be high or low. This is possible. But it is better to say you have little or much of it. It also can be absent. In German this is:
Georg hat viel Selbstvertrauen. Er traut sich viel zu.
Georg has a lot of self-confidence. He dares a lot.
(I know, my English translations are not always the best. I'm a German native speaker and just try to give you a hint to understand the German sentences)
Walter ist ängstlich und zaghaft. Er hat kein Selbstvertrauen.
Walter is frightened and timid. He has no self-confidence.