In what way is the following sentence grammatically incorrect? - "Il faut que je fais un effort."

Certain phrases in french trigger the subjunctive mood; 'il faut que...' is an example of this. In terms of grammar, this means that 'faire' must be conjugated differently to how you would usually conjugate it in the present tense. In this case, 'faire' is an irregular verb and so it doesn't follow the usual rule of taking the 3rd person plural conjugation, removing the ending, and adding the appropriate suffix. For this sentence you would need the 1st person singular (je) conjugation, which would be 'fasse'. Therefore due to the subjunctive being triggered, the grammatically correct sentence would be:

Il faut que je fasse un effort.

Meaning: It is necessary that I make an effort.