We would need the entire sentence and some context to answer that question.
My boss said that my English is unnatural when I used the following phrase "company experiences a lack of financing". Is it so?
The decline in sales amount can be explained by the fact that our customers experience a lack of financing. Therefore they tend to choose cheaper equipment.
In your complete sentence, I would use "are experiencing" instead of "experience". But in your first post you wrote "company experiences a lack of financing". That is much different from "customers experience a lack of financing".
This the type of "business writing" that gives business writers a bad name. There is no need to use "experience" in this sentence, even though it can certainly be grammatically correct.
Our customers are having trouble getting financing, so they are choosing less expensive equipment.
Our customers' inabilty to gain access to financing is causing them to make lower-cost choices.
(But if your competitors offer less expensive equipment that still meets customer needs, the lack of financing may not be what is driving your customers away.)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.