"Doctor Li is at the hospital."
There are 2 problems:
是 in 李医生是在医院 is redundant. It is not necessary in Chinese and we call this kind of sentence a "病句/sick sentence".
When we make a sentence A是B, B can only be a noun or an equivalent structure, i.e. a noun, pronoun or a noun phrase. 在医院 is not a noun phrase. To make it grammatically correct, we need to add at least a structural particle 的 at the end to make it a noun phrase, so 李医生是在医院的. It will then be a grammatically correct sentence (but as Doctor Li is not a plant that permanently resides inside the hospital, the meaning of the sentence is still wrong.)
是 is not necessary because 在 means "is at", so you're saying "is is at".
是 is used to identify, you cannot use it here to locate Dr. Li. That's what 在 is for.
Unlike English, in Chinese, you don't say 李医生 IS 在 医院。Simply 李医生在医院 is correct. You can think of 在 as "is at ..." if it helps.
The 是 is unnecessary. That's our English grammar showing. A direct translation of the correct sentence would be: "Dr. Li at hospital." Not: "Dr. Li is at hospital."
是 isn't usually used in Chinese like it is in english The literal translation of this is "Doctor Li at Hospital" as the "is" would be redundant in chinese.