Please see the accepted answer in the duplicate target.
In short, both are perfectly fine, with some difference in style. In Present-day English, the preterit – e.g., did, stood, took, played – can be used to denote modal remoteness, and this has extended over to was as well.
However, there's a single vestigial form of be (were) for a 1st/3rd person singular subject which is used to indicate modal remoteness. There are no other such forms. Assuming you're talking about a non-factual/unreal situation, if there were/was more time means "if I had more time".
(Apologies for having no imagination in the examples below.)
Here are a couple of examples that try and clarify what's meant by real and unreal situations using the sentence in the question:
Unreal situation in the present:
Kim is working hard on their project, which they need to finish before the clock strikes noon. Kim glances at the clock, which shows 11.55 a.m.
Kim: "If there were / was more time, I'd be able to complete the project." (=but there isn't any time left)
Unreal situation in the past:
Pat: "Why did you get a B on your exam?"
Kim: "If there had been /
were / was more time, I would've aced it." (=but there wasn't enough time)
Real situation in the present:
Pat enters the room and spots Kim and Bran playing games on Kim's PlayStation.
Pat: "Have you finished your project?"
Pat: "Wait, when's the deadline?"
Bran: "We've got until noon, I think."
Pat: "Have you given up? If there is /
were / was more time, why aren't you working on it?" (=there's still more time)
Real situation in the past:
Kim: "We didn't manage to finish the project."
Pat: "Oh, when did you get home?"
Kim: "I don't know. Three maybe?"
Pat: "And when was the deadline?"
Kim: "At six."
Pat: "If there
were / was more time, why didn't you finish the project?" (=there was more time)