x = rand(10,1);y = x + rand(10,1);
fit(x,y,'log10')
It is almost always, ALWAYS the case that when this happens, you were previously using a different, more recent MATLAB release.
As you can see above, it does work. But, to be honest, I did not recall that being one of the valid fit types. So I tried it on my computer, and it failed, using R2023a.
And that means they just introduced that option. So you were possibly using an online MATLAB, or a new release on some other computer. But today, you were using an older release, and the option did not exist then. They must have alloed that option in only the last year. I could check the release notes, but I am sure that is the case.
What can you do? Well, make sure your version is a sufficiently recent one. Otherwise, you could use a custom fittype of that form. For example, this would work, even in an older release:
mdl = fittype('a*log10(x) + b',indep = 'x')
fittedmdl = fit(x,y,mdl)
Note that because the model is actually linear in the parameters, fit was smart enough to not bother needing starting values for the 'log10' option.
