I hope you and your family are well.
I wanted to know, is it a halacha that one is not allowed to daven outside?
It is permitted to daven outside, however a person should not daven in an open place such as a field that doesn’t have a fence around it. The reason is because when a person is in an open place such as in an open field it can give the person a feeling of gaavah, (for the same reason we don’t daven in a high place) which is detrimental to one’s tefilla. Therefore davening in backyard or alleyway is fine. Even davening in the street will not have this issue, as long as you can concentrate while davening there. When davening in an open park the person should go between the trees to take away the feeling of being in an open place. If one is in such a place and doesn’t have another place to go he should still daven there.
Have a good Shabbos
Shulchan Aruch O:CH 90-5, M:B 90- 11,12.