Firstly I'd agree that there's not a significant far left political party in Scotland (Scottish Greens, perhaps?) but it is still more left-leaning overall. But re the "nationalist" bit - that word is overloaded. So the word usually means a couple of things:

1. "far-right beliefs that centre on religious and ethnic hatred"

2. or, "a desire to become an independent nation"

In this case when people say the SNP is nationalist it's the second - their policy is that Scotland should be an independent country. For what it's worth they seem to have a strong pro-immigration stance[0] and a belief in racial equality[1], which are usually things type #1 nationalists are heavily against - so take from that what you will.

[0] - https://www.snp.org/policies/pb-what-is-the-snp-s-policy-on-...

[1] - https://www.snp.org/policies/pb-what-actions-are-the-snp-tak...